This will fire on init
so you just need to refresh a page one time for it to work. I left out the actual value of $new_value
— you need to define that.
<?php
add_action( 'init', 'wpse_75308_update_video_meta' );
function wpse_75308_update_video_meta() {
// Get all posts
$posts = get_posts( array( 'numberposts' => -1 );
// Loop through each and update the custom field where it exists
foreach( $posts as $post ) {
update_post_meta( $post-ID, 'video_thumb', $new_value );
}
}