Follow the directions posted by Tererence Devlin in this post. Note the text in red. Also be aware you must have had some version of iPhoto 9.x installed prior to the upgrade to Yosemite in order to obtain iPhoto 9.6.1.
Terence Devlin
Apr 14, 2015 11:21 AM
Re: Is Iphoto gone ? i want it back!
in response to Johannes666
Recommended
Go to the App Store and check out the Purchases List. If iPhoto is there then it will be v9.6.1
If iPhoto 9.6.1 is there, close the App Store, drag your existing iPhoto app (not the library, just the app) to the trash and then go back to the App Store.
Install the App from the App Store.
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.