how do I update photo to iphoto

Your question is ambiguous, but I will take a stab.

Photos is the newer program. It replaced iPhoto several years ago. You can get Photos to convert an iPhoto Library. Launch Photos with the option key held down. from the resulting dialog, choose your iPhoto Library. Photos will create a new Photos Library.

If I take your question literally, there is no reverse process to go from Photos to iPhoto. You will have to export all the Photos images as originals and import them into your iPhoto library.