How to solve it-updating plugins in wordpress? -

i have made modifications in several of plugins in wordpress. these plugins outdated. have submit project client. client asking me update plugins. problem when update plugins, changes made in plugins have gone away. so, want ask there concept of child plugin in wordpress , if so, how create it; changes remain intact.

i don't know plugins using giving example of woocommerce delivery date plugin. have disabled updates plugin , have used below code achieve :

/* function remove plugin update notices – woocommerce delivery date*/ function disable_plugin_updates( $value ) {    unset( $value->response['woocommerce-delivery-date/woocommerce-delivery-date.php'] );    return $value; } add_filter( 'site_transient_update_plugins', 'disable_plugin_updates' );