I am entirely new to Git, SourceTree, and everything about it. I am using SourceTree with Bitbucket but I need some help. As I know absolutely nothing about Git or SourceTree, I would like to learn how to use SourceTree to revert all the files in my Git repository to the state they were in when I made an earlier commit. Basically I want to roll back to an earlier commit so that I can access the files as they were at that point, but I want to be able to keep my existing commits so that I can roll forward as necessary. I also want to be able to make commits of the files based on the files from older commits. How can I do all of this?