How do I force homebrew to update installed software?

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

Solution 1:

I don't know why this happens with Homebrew when you don't do anything to mess with it, but here's how I fixed it:

1. Change to your Homebrew cellar directory cd `brew --cellar`
2. Reset git repository git reset --hard HEAD
3. Upgrade pianobar, at last! brew upgrade pianobar

FYI, brew update will update Homebrew to the latest version, so you may want to do that before step 3.

Solution 2:

As @Felix said, use upgrade not update.

If the problem comes from your dirty git tree and you just want to forget about your modifications, type git reset --hard HEAD before upgrading.