Change set featured image box text (Wordpress), just paste it to functions.php

change featured image box text

function custom_admin_post_thumbnail_html( $content ) {
    return $content = str_replace( __( 'Set featured image' ), __( 'Set thumbnail image' ), $content);
}
add_filter( 'admin_post_thumbnail_html', 'custom_admin_post_thumbnail_html' );