How do I reset the git master branch to the upstream branch in a forked repository?

You can reset your local master branch to the upstream version and push it to your your 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.)

This would reset your master branch with the upstream master and if the branch has been updated since your forked it would pull those changes as well.

git checkout master 
git reset upstream/master
git pull --rebase upstream master
git push origin master --force

PS: Assuming Upstream is the original repo while origin is your copy.