Switch to Photos from iPhoto on Mac
If you didn’t update to OS X 10.10.3 or later, the Photos app is installed automatically when you update your version of macOS.
When you first open Photos on your Mac, your photo library is upgraded. The upgraded library is compatible with both Photos and iPhoto (if installed), but any changes you make in Photos won’t appear in iPhoto, and vice versa.
If you previously had iPhoto on your Mac, it’s still on your Mac. You can open it from Launchpad or from the Applications folder in the Finder. However, it’s strongly recommended that you use Photos instead of iPhoto.
For more information about the Photos app, see the Photos User Guide.