Updating / Upgrading Mac

Hi, I have a iMac running macOS High Sierra (late 2013 model) and a iPhone 14. The phone is no longer seen when plugged in. How can I update/upgrade the mac software? I don't have any updates listed as available for the operating system only for pages, numbers etc. but can't install those as I don't have the right version operating system. I'm totally confused! Many thanks for reading and hopefully helping.