I have mac mini 2012; El Capitan 10.11.6 what/how can I update?

It can be updated to Catalina or earlier

See the following from Identify your Mac mini model - Apple Support

Mac mini (Late 2012)

Model Identifier: Macmini6,1; Macmini6,2

Part Numbers: MD387xx/A; MD388xx/A, MD389xx/A

Newest compatible operating system: macOS Catalina 10.15.7

See instructions in How to download macOS - Apple Support

Note that if you update to Catalina you will also need to update any non-Apple 32-bit apps that you have. See: https://www.macworld.com/article/232735/how-to-check-if-mac-software-is-32-or-64-bit.html