Wordpress Update Plugin Hook/Action? Since 3.9

Since WordPress 3.9 you can use upgrader_process_complete hook.
This hook will be fire when upgrader process is complete (plugins and themes are updated).

See reference 1, 2, 3

Here is an example code:

<?php 
/**
 * Plugin Name: Test plugin 1
 * Plugin URI: https://rundiz.com
 * Description: A very simple plugin for testing. This plugin do nothing.
 * Version: 0.1.8
 * Author: Vee Winch
 * Author URI: http://rundiz.com
 * License: MIT
 * License URI: https://opensource.org/licenses/MIT
 * Text Domain: test-plugin1
 * Domain Path: 
 */


add_action('upgrader_process_complete', 'testplugin_upgrade_completed', 10, 2);
/**
 * Upgrader process complete.
 *
 * @see \WP_Upgrader::run() (wp-admin/includes/class-wp-upgrader.php)
 * @param \WP_Upgrader $upgrader_object
 * @param array $hook_extra
 */
function testplugin_upgrade_completed(\WP_Upgrader $upgrader_object, $hook_extra)
{
    // get current plugin version. ( https://wordpress.stackexchange.com/a/18270/41315 )
    if(!function_exists('get_plugin_data')){
        require_once(ABSPATH . 'wp-admin/includes/plugin.php');
    }
    // https://developer.wordpress.org/reference/functions/get_plugin_data/
    $plugin_data = get_plugin_data(__FILE__);
    $plugin_version = ($plugin_data['Version'] ?? 'unknown.version');
    unset($plugin_data);

    if (
        is_array($hook_extra) && 
        array_key_exists('action', $hook_extra) && 
        $hook_extra['action'] == 'update'
    ) {
        if (
            array_key_exists('type', $hook_extra) && 
            $hook_extra['type'] == 'plugin'
        ) {
            // if updated the plugins.
            $this_plugin = plugin_basename(__FILE__);
            $this_plugin_updated = false;
            if (array_key_exists('plugins', $hook_extra)) {
                // if bulk plugin update (in update page)
                foreach ($hook_extra['plugins'] as $each_plugin) {
                    if ($each_plugin === $this_plugin) {
                        $this_plugin_updated = true;
                        break;
                    }
                }// endforeach;
                unset($each_plugin);
            } elseif (array_key_exists('plugin', $hook_extra)) {
                // if normal plugin update or via auto update.
                if ($this_plugin === $hook_extra['plugin']) {
                    $this_plugin_updated = true;
                }
            }
            if ($this_plugin_updated === true) {
                // if this plugin is just updated.
                // do your task here.
                // DON'T process anything from new version of code here, because it will work on old version of the plugin.
                // please read again!! the code run here is not new (just updated) version but the version before that.
                file_put_contents(WP_CONTENT_DIR . '/test.txt', 'v'.$plugin_version."\r\n", FILE_APPEND);
                // set transient to let it run later.
                set_transient('testplugin_just_updated', 1);
            }
        } elseif (
            array_key_exists('type', $hook_extra) && 
            $hook_extra['type'] == 'theme'
        ) {
            // if updated the themes.
            // same as plugin, the bulk theme update will be set the name in $hook_extra['themes'] as 'theme1', 'theme2'.
            // normal update or via auto update will be set the name in $hook_extra['theme'] as 'theme1'.
        }
    }// endif; $hook_extra
}// testplugin_upgrade_completed


add_action('plugins_loaded', 'testplugin_pluginloaded');
/**
 * Run once plugin loaded (on every page load).
 */
function testplugin_pluginloaded()
{
    // get current plugin version. ( https://wordpress.stackexchange.com/a/18270/41315 )
    if(!function_exists('get_plugin_data')){
        require_once(ABSPATH . 'wp-admin/includes/plugin.php');
    }
    // https://developer.wordpress.org/reference/functions/get_plugin_data/
    $plugin_data = get_plugin_data(__FILE__);
    $plugin_version = ($plugin_data['Version'] ?? 'unknown.version');
    unset($plugin_data);

    if (get_transient('testplugin_just_updated') && current_user_can('manage_options')) {
        // if it was marked in transient that this plugin just updated and current user is admin.
        // you can use new verion of code here.
        file_put_contents(WP_CONTENT_DIR . '/test-update-by-transient.txt', 'v'.$plugin_version."\r\n", FILE_APPEND);

        // your update code here.
        
        // delete transient when done to do not let this code run again.
        delete_transient('testplugin_just_updated');
    }
}// testplugin_pluginloaded

The upgrader_process_complete hook will be run with your current version of code while the plugin/theme is updating. It is not use new version.

Scenario

1. You have plugin version 1.0
2. You run the update page or auto update.
3. Your plugin version 2.0 will be downloaded and extract. The upgrader_process_complete hook will be called.
4. Your plugin version 1.0 will be run in upgrader_process_complete hook.
5. Once done, reload your page and the plugins_loaded hook will be called.
6. Your plugin version 2.0 run in plugins_loaded hook. (The plugin must be activated.)

These are already explained in the code I have posted from earlier (before edit) but maybe not clearly or hard to see it.

The upgrader_process_complete hook is created for this (please read in the reference link 3). To run after upgrade completed.

You may use plugins_loaded hook with the code in accepted answer. It did work and write shorter or you may have any better idea to use with upgrader_process_complete hook.

The upgrader_process_complete hook will be work when:

1. Update via the update page.
2. Update via the plugins or themes page.
3. Update via auto update or WP Cron.

The code above doesn't work when you update plugin or theme via FTP because it can't detected transient option. In this case, the accepted answer is the only best option for you.

You can hook into the upgrader_pre_install and upgrader_post_install filters.

From the discussion where they decided not to add a custom hook/function specific to upgrade, it sounds like "most people" (as of 4 years ago) use register_activation_hook, since it's called when a plugin is upgraded through the admin page; most examples I've seen since then follow that trend.

For most usage I would suggest not hooking through plugins_loaded, as it would get called on every page load. The exception to this is mentioned in the discussion: upgrade paths via FTP/SVN are 'edge cases', since WP wouldn't have a mechanism to know that the plugin was changed, in which case the previous answer might be more relevant.

See https://gist.github.com/zaus/c08288c68b7f487193d1 for a 'simple framework' example using register_activation_hook.

I don't think an action has been added. You can look at version details for any version and see any new actions added.

The WordPress Way to run code on plugin update is what is described here:

The proper way to handle an upgrade path is to only run an upgrade procedure when you need to. Ideally, you would store a “version” in your plugin’s database option, and then a version in the code. If they do not match, you would fire your upgrade procedure, and then set the database option to equal the version in the code. This is how many plugins handle upgrades, and this is how core works as well.

and with code example here:

function myplugin_update_db_check() {
    global $jal_db_version;
    if (get_site_option( 'jal_db_version' ) != $jal_db_version) {
        jal_install();
    }
}
add_action( 'plugins_loaded', 'myplugin_update_db_check' );