Rapid Environment Editor is a free application for Windows specialized in addition, edition and deletion of those variables as well as listing the different system variables in the environment.
It's not difficult to use if you know what you are working on, choose the name and the kind of variable you want to edit and give it a value. You'll automatically see the variable in the list.
CAUTION, only use this application if you know what you are working, on. The bad use of this application may cause non-desired behaviour of your computer.