Android Studio Instructions: if you want to do this in Android Studio, press alt + 9 (or Command + 9 on Mac) to open the Version Control panel. Switch to the Log tab and right click on a previous commit. Select Checkout Revision.
Command line instructions: Open the command line tool you are using.
Go to the Android app’s Git directory (using cd).
Execute git log and find the previous commit you want to revert to.
commit 7c247be6d8975dc88f6cc2631c154786a1f3b79e
Author: John Doe <john@doe.ca>
Date: Fri Jun 11 22:37:35 2015 -0400
Some helpful commit message should be here.
If that is the commit you want to revert to, then execute git checkout 7c247b.