How do I force homebrew to update installed software?

Question

I have pianobar installed and I'm trying to update it using brew update pianobar. I also tried brew update pianobar --force but that didn't have any effect. It says that I have local changes, but I'm not sure how to tell it to just overwrite these.

From http://github.com/mxcl/homebrew
 * branch            master     -> FETCH_HEAD
error: Your local changes to the following files would be overwritten by merge:
    Library/Formula/android-ndk.rb
    Library/Formula/android-sdk.rb
    Library/Formula/center-im.rb
    Library/Formula/putty.rb
    Library/Formula/python.rb
    ... (lines removed) ...
    Library/Formula/upx.rb
    Library/Formula/vimpager.rb
    Library/Homebrew/cmd/doctor.rb
Please, commit your changes or stash them before you can merge.
error: The following untracked working tree files would be overwritten by merge:
    Library/Formula/tth.rb
Please move or remove them before you can merge.
Updating 45bc7ad..36f4400
Aborting
Error: Failed while executing git pull http://github.com/mxcl/homebrew.git master