Git is a backup system itself so after accidental deletion or modifications it will be able to bring the files back. If you still want a physical backup, you can just copy the .git directory to a different location. For more information about how Git works, you can read Git backups.
As for undo changes, you can reverse a commit in Sourcetree by right-clicking on the commit.
If you were referring to something different, please let us know and we'll be happy to continue helping you.
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