github fork forced sync

git remote add upstream https://github.com/some_user/some_repo
git fetch upstream
git checkout master
git reset --hard upstream/master  
git push origin master --force

Keep in mind, that this makes your repo look exactly like upstream - if you have commits, they will be eaten! If you want to update to upstream (but still keep your commits), you want git merge upstream/master. If you want to start a new patch based on upstream, the best way to do it is git checkout -b my-new-feature upstream/master

Of course, this all expects you have a remote named upstream, which you can create by doing git remote add upstream https://[upstream guy's repo url]
(Just to clarify, the term 'upstream' means the person you forked from)

`git push origin master --force` pushes back YOUR repo master branch.

See also:
https://stackoverflow.com/questions/9646167/clean-up-a-fork-and-restart-it-from-the-upstream/39628366