Git Update

Add the remote, call it upstream:

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

I also always make sure upstream is configured in my .git/config like this to avoid accidental pushes to upstream that bypass pull requests:

[remote "upstream"]
	url = https://github.com/whoever/whatever.git
	fetch = +refs/heads/*:refs/remotes/upstream/*
	pushurl = no_push

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