N.B. If you simply want to move a branch to another commit, the easiest way is
git branch -f branch-name new-tip-commit
as detailed by Chris Johnsen.
You can do it for arbitrary refs. This is how to move a branch pointer:
git update-ref -m "reset: Reset <branch> to <new commit>" refs/heads/<branch> <commit>
The general form:
git update-ref -m "reset: Reset <branch> to <new commit>" <ref> <commit>
You can pick nits about the reflog message if you like - I believe the branch -f one is different from the reset --hard one, and this isn't exactly either of them.
