How do you switch branches in a bare repo?

Hi all,

I want to remove some commits (with git reset) from a bare repo, but they're not in the master branch and I can't figure out how to change branches:

$ git checkout newbranch fatal: This operation must be run in a work tree

I've tried running the reset locally and pushing the change (with -f) but that doesn't work either, and Google isn't being much help.

How do you switch between branches in a bare repo?

Many thanks, Adam.

(Please CC)