It depends on how you installed it.
The best way to uninstall Installer.app is to use iBrickr (Windows). Go to Applications and check to see if the app is listed within the iPhone image on the right. If it is, you can delete it immediately. If not, go to the Browse Applications list and install Installer 2.63 from there. Once your phone soft resets, you can use iBrickr to delete the new installation (which of course also deletes the overwritten old one).