So, are you on a git repository with a remote repo named heroku?
Have you made your changes and then click? They need to be localized so that they can be clicked.
When cloning a repository using git, the remote repository from which you cloned will be added as a "source", then simply pressing git will lead to the origin. If this is not the case, you can use the name of the other / remote repository that you added first, or its URL.
Also see the doc for git push: http://www.kernel.org/pub/software/scm/git/docs/git-push.html
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