Note that you must have some version of iPhoto 9.x for this to work. While you still cannot update iPhoto, you can re-install the app which will download the current version. Support for iPhoto was dropped with the release of Photos in Yosemite 10.10.3. If you didn't update your 9.x version of iPhoto prior to updating OS X Yosemite to 10.10.3, you cannot update the app through the App Store.
