Just ran into this as I had changed my account password a few days back, and the approach to change your password for your git client on OS X is not that obvious. At the same time I enabled Two Factor Authentication for my github account. When using with a command-line git client, this also requires generating a Personal Access Token and passing it with your authentication.
Instructions in this post here – you need to change your password in the Keychain Access app.
Create a Personal Access Token following steps here.
Next, when adding a remote for the repo, pass the token like this:
git remote add github https://github-id:firstname.lastname@example.org/path-to/remote-repo.git
Now when you push to the remote repo, you’ll be authenticated with your token, and should be all set.