You should now have a local clone. To make change, the first thing you need to do is access the correct version of the source code by navigating to the correct branch in your Mercurial/Git client. Then you can make your modifications.
In our example, the Unity Test Tools repo is new and has only one commit on the stable branch. Here is an example of updating explicitly to that version:
Once you have the correct version of the source code, you can make your modifications as you see fit. When you go back to your Mercurial/Git client, it will detect that you have modified files:
You can see them in the Working Copy view. Click Commit to commit these changes:
Next, push the changes. Your client will show you how many changesets you have waiting to be pushed:
Click Push to push your changes: