I do not want to upload my app to the wrong domain.
How can I change the git master branch on git?
You can have as many branches you want, just as a regular git repository, but according to heroku docs, any branch other than master will be ignored.
http://devcenter.heroku.com/articles/git
Branches pushed to Heroku other than master will be ignored. If you’re working out of another branch locally, you can either merge to master before pushing, or specify that you want to push your local branch to a remote master.
This means that you can push anything you want, but you app at heroku will always point to the master branch.
But, if you question regards how to create branches and to work with git you should check this other question