How to fetch Data in WordPress using MySQLi or $wpdb

To fetch data from database table

$results = $wpdb->get_results( "SELECT * FROM $table_name"); // Query to fetch data from database table and storing in $results
if(!empty($results))                        // Checking if $results have some values or not
{    
    echo "<table width='100%' border='0'>"; // Adding <table> and <tbody> tag outside foreach loop so that it wont create again and again
    echo "<tbody>";      
    foreach($results as $row){   
    $userip = $row->user_ip;               //putting the user_ip field value in variable to use it later in update query
    echo "<tr>";                           // Adding rows of table inside foreach loop
    echo "<th>ID</th>" . "<td>" . $row->id . "</td>";
    echo "</tr>";
    echo "<td colspan='2'><hr size='1'></td>";
    echo "<tr>";        
    echo "<th>User IP</th>" . "<td>" . $row->user_ip . "</td>";   //fetching data from user_ip field
    echo "</tr>";
    echo "<td colspan='2'><hr size='1'></td>";
    echo "<tr>";        
    echo "<th>Post ID</th>" . "<td>" . $row->post_id . "</td>";
    echo "</tr>";
    echo "<td colspan='2'><hr size='1'></td>";
    echo "<tr>";        
    echo "<th>Time</th>" . "<td>" . $row->time . "</td>";
    echo "</tr>";
    echo "<td colspan='2'><hr size='1'></td>";
    }
    echo "</tbody>";
    echo "</table>"; 

}

NOTE: Change your data fetching format according to your need (table structure)

To update time field on if condition

 if($userip==245.356.134.22){  //Checking if user_ip field have following value
$wpdb->update( 
$table_name, 
array( 
    'time' => 'YOUR NEW TIME' // Entring the new value for time field
),      
array('%d')                   // Specify the datatype of time field
);
}

Update

If you want to check if the IP you are going to insert in database is already exist or not then check it like this

global $wpdb,$ip;
$results = $wpdb->get_results( "SELECT user_ip FROM $table_name");  //query to fetch record only from user_ip field

$new_ip = 245.356.134.22;   //New Ip address storing in variable

if(!empty($results))                       
{    
    foreach($results as $row){  
    $old_ip = $row->user_ip;        // putting the value of user_ip field in variable
    if($new_ip==$old_ip){           //  comparing new ip address with old ip addresses
      $ip = 'Already Exist';        // if ip already exist in database then assigning some string to variable
    }
    }

}
if($ip = 'Already Exist'){          // Checking if variable have some string (It has some string only when if IP already exist in database as checked in if condition by comparing old ips with new ip)
//Insert query according to Ip already exist in database
}else{
//Insert query according to Ip doesn't exist in database
}

You indicate MYSQLi in your question, but label and refer to MySQL in your question. If you are using MySQL the following will work for you:

This is fairly simple, and given the fact that you already have the insert statement constructed, you can just use update like so:

$wpdb->update($table_name , array('user_ip' => $user_ip, 'post_id' =>$postID, 'time' => $visittime),array('%s','%d', '%d') );

Then all you would have to do is set the values in your query as to what data in the DB you want to change. You can find examples here

As for fetching all of the data you could use the follow:

$results = $wpdb->get_results("SELECT * FROM table_name"); 

You can add any WHERE parameters to it or sort it just like and standard SQL query statement. You can just then loop through is with a foreach loop and then you would just echo out your data you retrieved.

//For fetching data use
global $wpdb;
$results = $wpdb->get_results("SLECT * FROM table_name"); 
//and for update use below code 
$wpdb->update( 
  $table_name, 
  array( 
    'time' => time(),   // string
  ), 
  array( 'user_ip' => '245.356.134.22' ), 
  array('%s'), 
  array( '%d' )
);