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