Upgrade Imac operating system
I have a late 2009 Imac with Sierra 10.12.6 installed.
How can I upgrade to a newer System that includes "Photos"
The furthest you can go on a 21.5" or 27" 2009 iMac is to High Sierra 10.13.6. Beginning with Mojave (10.14.x) it required a GPU with Metal. Metal only became available in iMac GPUs in 2012 and later. And before you ask, no cannot retrofit or upgrade your GPU to have Metal. In short, if you want to stay current it may be time to consider retiring the old Mac (it's 11-12 years old now) and buy a new iMac.
If you are considering doing that, please ensure all of your apps are 64 bit apps as the new version of Mac OS (Catalina 10.15.x) will NOT run 32 bit apps and requires 64 bit applications.