How do I update my iMac?

An iMac 11,2 model is a 2010 model and it cannot be upgraded beyond High Sierra 10.13.6. That is the last update/upgrade possible.

Also see the following from Identify your iMac model - Apple Support

iMac (21.5-inch, Mid 2010)

Model Identifier: iMac11,2

Part Number: MC508xx/A, MC509xx/A

Newest compatible operating system: macOS High Sierra

If you continue to use it, for security reasons you should consider using the following 115 version of Firefox instead of Safari: https://support.mozilla.org/en-US/kb/firefox-users-macos-1012-1013-1014-moving-to-extended-support#firefox:mac:fx115

What type of Mac (iMac, MacBook Air, etc) and what model year is it? Both of those can be found at Apple menu > About this Mac.