The fastest way to restore an old version is to use the "reset" command:
$ git reset --hard 0ad5a7a6
This will rewind your HEAD branch to the specified version. All commits that came after this version are effectively undone; your project is exactly as it was at that point in time.
The reset command comes with a couple of options, one of the more interesting ones being the "--soft" flag. If you use it instead of --hard, Git will keep all the changes in those "undone" commits as local modifications:
$ git reset --soft 0ad5a7a6
You'll be left with a couple of changes in your working copy and can then decide what to do with them.
In case you are using the Tower Git client, you can use the reset command right from a commit's contextual menu. And in case you made a mistake: simply hit CMD+Z to undo the reset and restore the removed commits!
Restoring a Revision in a New Local Branch
As said, using the reset command on your HEAD branch is a quite drastic action: it will remove any commits (on this branch) that came after the specified revision. If you're sure that this is what you want, everything is fine.
However, there is also a "safer" way in case you'd prefer leaving your current HEAD branch untouched. Since "branches" are so cheap and easy in Git, we can easily create a new branch which starts at that old revision:
$ git checkout -b old-project-state 0ad5a7a6
Normally, the checkout command is used to just switch branches. However, providing the -b parameter, you can also let it create a new branch (named "old-project-state" in this example). If you don't want it to start at the current HEAD revision, you also need to provide a commit hash - the old project revision we want to restore.
Voilà: you now have a new branch named "old-project-state" reflecting the old version of your project - without touching or even removing any other commits or branches.
In case you are using the Tower Git client, you can simply drag & drop the commit you want to return to and create a new branch: