How do I upgrade to photo?

There is no upgrade needed. The Photos.app is a part of the system and comes preinstalled on any Mac with OS X 10.10.3 or later. You just have to launch Photos while holding down the options key ⌥ and select your iPhoto Library in the Library chooser dialog.

Photos will create a new library from your iPhoto Library, where you can access the photos and videos from your iPhoto Library.

You may want to have a look at the Photos User Guide to get started (click ont the "Table of Contents" line):

Photos User Guide for Mac - Apple Support