You can reset your local master branch to the upstream version and push it to your origin repository.
Assuming that "upstream" is the original repository and "origin" is your fork:
# ensures current branch is master
git checkout master
# pulls all new commits made to upstream/master
git pull upstream master
# this will delete all your local changes to master
git reset --hard upstream/master
# take care, this will delete all your changes on your forked master
git push origin master --force
(You can define the original repo as "upstream" with git remote add upstream /url/to/original/repo.)