To force your forked repo to match upstream’s master

1git remote add upstream https://github.com/some_user/some_repo
2git fetch upstream
3git checkout master
4git reset --hard upstream/master  
5git push origin master --force

Source: https://gist.github.com/glennblock/1974465

For syncing tags

1git fetch --tags upstream
2git push -f --tags origin master