Go into the PHP file for the plugin and change the information in the header comment. For example, this is the one from the Hello Dolly plugin that comes with WordPress:
/**
* @package Hello_Dolly
* @version 1.6
*/
/*
Plugin Name: Hello Dolly
Plugin URI: http://wordpress.org/plugins/hello-dolly/
Description: This is not just a plugin, it symbolizes the hope and enthusiasm of an entire generation summed up in two words sung most famously by Louis Armstrong: Hello, Dolly. When activated you will randomly see a lyric from <cite>Hello, Dolly</cite> in the upper right of your admin screen on every page.
Author: Matt Mullenweg
Version: 1.6
Author URI: http://ma.tt/
*/
Change the information in here to reflect that it is a “new” plugin. Just make sure you keep any copyright notices and such intact.