Delete it and reinstall:
To re-install iPhoto
1. Put the iPhoto.app in the trash (Drag it from your Applications Folder to the trash)
2. Download it from the App Store to reinstall It's on your Purchases List* there.
For older versions that have been installed from Disk you'll need these additional steps:
2a: On 10.5: Go to HD/Library/Receipts and remove any pkg file there with iPhoto in the name.
2b: On 10.6: Those receipts may be found as follows: In the Finder use the Go menu and select Go To Folder. In the resulting window type
/var/db/receipts/
2c: on 10.7 or later they're at
/private/var/db/receipts
A Finder Window will open at that location and you can remove the iPhoto pkg files.
3. Re-install.
If you purchased an iLife Disk, then iPhoto is on it.
If iPhoto was installed on your Mac when you go it then it’s on the System Restore disks that came with your Mac. Insert the first one and opt to ‘Install Bundled Applications Only.
*Sometimes iPhoto is not visible on the Purchases List. it may be hidden. See this article for details on how to unhide it.
http://support.apple.com/kb/HT4928
One question often asked: Will I lose my Photos if I reinstall?
iPhoto the application and the iPhoto Library are two different parts of the iPhoto programme. So, reinstalling the app should not affect the Library. BUT you should always have a back up before doing this kind of work. Always.
