iMac 27 inch, late 2013

Back up your data, check your applications for compatibility, and click on Mojave or Catalina.

(206799)

Just to add, Catalina is 64bit only so if you still have 32bit software installed you would have to update the software or find a replacement.

You can check for 32bit apps with Go64.

https://www.stclairsoft.com/Go64/