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I'd say the best way would be to do a restore in itunes. You might want to back up your camera roll pictures first, because I think I lost mine when I had to restore last time.
 
how to uninstall the installer

i have to agree with the poster above. doing a restore will revert your iphone to factory settings without the installer. it's easy and relatively fast. you will lose, however, your camera roll and your youtube bookmarks if you restore from a backup file.
 
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).
 
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