You can also change an environment variable to something else, as well as unset it so it isn't in your environment at all. You can't set an environment variable to nothing ("") to remove it. You have to use a command.
For the price of most other course's PDFs only, you can get the full PDF for this class and 2 videos demonstrating the whole book for both Unix/OSX Terminal and Windows PowerShell. The course is self-paced so you can go through it any time you want, as many times as you want.