As far as I can tell, GitHub does not provide shell access, so I’m curious about how you managed to log in in the first place.
$ ssh -T git@github.com
Hi username! You've successfully authenticated, but GitHub does not provide
shell access.
You have to clone your repository locally, make the change there, and push the change to GitHub.
$ git clone git@github.com:username/reponame.git
$ cd reponame
$ git mv README README.md
$ git commit -m "renamed"
$ git push origin master