i have photos 5 how can i update? bought mac in 2015

Why do you think you need to update Photos? What is the problem? Is something not working as expected?

• If you are seeing a message, that Photos cannot open a library and needs to be updated, you may not be running the most recent version of macOS 10.15. Then install any waiting system update.
• If Photos is crashing and you get a message suggesting to upgrade, try to find out, why Photos is crashing.You may want to post the exact error message here.

That is the latest version.

It comes as part of MacOS, and is updated when you update that. 5 is the version that comes with Catalina