How do I access old photos from iCloud?

If you have iCloud Photos turned on, the photos on the device are the same as on iCloud, with the possible exception that some of them may be space-saving optimized versions if you have that option turned on. You view them in the Photos app on an iOS device. You can also use a computer (or force a mobile browser to access the desktop site) and login to icloud.com and go to the Photos app there. See: Set up and use iCloud Photos - Apple Support

If you didn't have iCloud Photos turned on but do have an iCloud backup, you can restore the entire phone (not just the photos) from iCloud but you cannot otherwise view the contents of the backup. iCloud: Restore or set up iOS devices from an iCloud backup