In Windows you can edit your system's environment variables by opening up the system properties window by navigating to Control Panel\System and Security\System --> and clicking on "Advanced system settings" in the left pane. Then click on "Environment Variables".
My prefered method is to just do a start menu -- or metro screen search -- by typing "environment". You are usually given a choice of editing the "account" variables, or the "system's".
Find the variable in question and edit/delete as you wish.
Only do this if you are sure of the consequences.