GIT is specifically designed to answer these questions. Unfortunately, it does take some time to learn to use appropriately.
The short answer, for how to create versions of your project is this:
Whenever you make a few changes, and have your project in a working state, you should "commit" those changes (right click on your app in the outline view).
If you want to copy those changes to GitHub.com (to make them available online, and provide online backup in case your computer crashes), then you also need to "push" them.
To restore an earlier version takes a bit more work...
The longer version is: read more about using Git, and/or ask one of your classmates/teammates who has used it before.
We'll be talking more about git soon.