how to revert to old iphoto

On OS X Yosemite (10.10.5) you need to update to iPhoto 9.6.1.

If you already have this version installed, drag it back from the Applications folder to the Dock and launch it.

If you still need to install the update, you can only update by reinstalling from your Purchases history at the App Store.

Barney-15E's user tip explains how to update to iPhoto 9.6.1 by reinstalling your App Store purchase.

See this link: Get iPhoto 9.6.1 if you didn't update before OS X 10.10.3

Basically, move the incompatible iPhoto version from the Applications folder to Trash (the application, nit your iPhoto LIbrary). Then sign into the AppStore and open the purchases tab. It should now show "Install" to the right of iPhoto.

You're welcome 🙂

Any ideas on how I can keep my photo books forever the way they were in this last iPhoto version?

Keep a backup copy of the iPhoto application and your current iPhoto library. The best would be a bootable clone of your system drive on an external HD. Ypu cn create bootable clones with utilities like Supr Duper orCarbon Copy Cloner. This way you will always be able to launch your Mac with the current system, just like it is now, even if you should accidentally upgrade to a system version that no longer supports iPhoto.

but I believe the oldest versions always get overwritten with new backups,

yes, when the drive fills up, the oldest versions may be deleted. And it is alwas safer to have two different backups. With only one backup you may notice too late, if your only backup is not readable.

Also, The bootable clone will let you revert to exactly the MacOS X version on the clone. With Time MAchine you can revert to earlier versions of the applications, but the options to revert to earlier system versions are limited, see: OS X Yosemite: Revert to a previous OS X version

And you cannot boot the Mac from a Time Machine backup. You first have to restore it.

I once had a problem during a thunder storm, where a lightning bolt caused a power surge, that damaged both my mac and the backup drive, because I was just updating the backup. I was really glad to have a second backup!