Whenever there is a plugin upgrade WordPress will display a notification in your plugins admin.
If for any reason you do not want to upgrade and wants to disable the notification, Just change the version number in the plugin’s main php file.
For example, if you do not want to upgrade Video Sidebar Widgets to version 2.1, as this version is only an inclusion of support for Google Videos and you do not need it. Just open video-sidebar-widgets.php using the plugin editor and change the version number to a greater number. The version number can be found on the top of the plugin file.
/* Plugin Name: Video Sidebar Widgets Plugin URI: http://denzeldesigns.com/wordpress-plugins/video-sidebar-widgets/ Version: 2.0 Description: A video sidebar widget using WordPress 2.8 Widgets API to display videos such as Vimeo, YouTube, MySpace Videos etc. Requires at least WordPress 2.8.1. Now including Random Video Sidebar Widget to randomly display 1 out of 5 preset video. Author: Denzel Chia Author URI: http://denzeldesigns.com/ */
Just change Version 2.0 to 4.0 and the notification will disappear.