How do I change the initial quantity of a product in woocommerce?

There is a beautiful example in WooCommerce docs
the example even includes a conditional amount.

/**
 * Adjust the quantity input values
 */
add_filter( 'woocommerce_quantity_input_args', 'jk_woocommerce_quantity_input_args', 10, 2 ); // Simple products

function jk_woocommerce_quantity_input_args( $args, $product ) {
    if ( is_singular( 'product' ) ) {
        $args['input_value']    = 2;    // Starting value (we only want to affect product pages, not cart)
    }
    $args['max_value']  = 80;   // Maximum value
    $args['min_value']  = 2;    // Minimum value
    $args['step']       = 2;    // Quantity steps
    return $args;
}

Ref: https://woocommerce.com/document/adjust-the-quantity-input-values/

what you really need to set is the 'input_value' (meaning quantity) value. you have to change the min / max value or step values.. those control the actually input values.

Using this code you can easy to set update quantity . qty box. i was also try this method it's working fine for me ...I hope it's also helpful thank you :)

function wpse_292293_quantity_input_default( $args, $product ) {


$productID = $product->id;

foreach( WC()->cart->get_cart() as $key => $item ){
    
    if( $item['product_id'] == $productID ){    
        $args['input_value'] = $item['quantity'];   
        return $args;       
    }
    
} 

$args['input_value'] = 1;
return $args;

}
 add_filter( 'woocommerce_quantity_input_args', 'wpse_292293_quantity_input_default', 10, 2 );

You can use the woocommerce_quantity_input_args filter:

function wpse_292293_quantity_input_default( $args, $product ) {
    $args['input_value'] = 2;

    return $args;
}
add_filter( 'woocommerce_quantity_input_args', 'wpse_292293_quantity_input_default', 10, 2 );

When the quantity input is output, it's output with the woocommerce_quantity_input() function. This function accepts several arguments including the default value for the input. This filter lets you replace values in the arguments across all uses of the function.