Change GIT branch in Unity

I’m using GitHub, SourceTree and Unity 5.2 Personal Edition.
My Git repository has a few branches, but I just can’t find the option in Unity how to switch to another branch?

Unity doesn’t have anything to do with Git.

You have to do these operations with your Git client.
If you are in Windows I recomend you TortoiseGit. Check the documentation for basic operations.