Add the remote, call it "upstream":

git remote add upstream https://github.com/whoever/whatever.git

Fetch all the branches of that remote into remote-tracking branches, such as upstream/master:

git fetch upstream

Make sure that you're on your master branch:

git checkout master

Rewrite your master branch so that any commits of yours that aren't already in upstream/master are replayed on top of that other branch:

git rebase upstream/master