Photos back to iPhoto?
iPhoto is a separate application from Photos (though if you had iPhoto on your dock, the 10.10.3 upgrade replaced its icon with Photos). You can still run it by using Spotlight or going to your Applications folder.

When you do run it, it may ask if you actually meant to open it, or if you meant to open Photos - choose the option to continue opening iPhoto. Your iPhoto library is still there and has not been removed. Keep in mind any changes made to your iPhoto library will not be reflected in Photos, so if you decide later that you do want to use Photos, you will need to create a new library in Photos to convert your iPhoto library at that time.