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.)