Make an existing Git branch track a remote branch?

Question

I know how to make a new branch that tracks remote branches, but how do I make an existing branch track a remote branch?

I know I can just edit the

Answer 1

For Git versions 1.8.0 and higher:

Actually for the accepted answer to work:

git remote add upstream 
git fetch upstream
git branch -f --track qa upstream/qa
# OR Git version 1.8.0 and higher:
git branch --set-upstream-to=upstream/qa
# Gitversions lower than 1.8.0
git branch --set-upstream qa upstream/qa