Hi, i would like to know how to update OS on iMac (27-inch, Late 2013), could do help? Thanks!

Back up your data, check your applications for compatibility, and click on El Capitan, High Sierra, Mojave, or Catalina. That model isn't capable of Big Sur.

(197924)

I stopped at Mojave on my Late-2013 27-inch iMac as it still supports 32-bit applications, is far more stable than my experience with Catalina, and has fewer operating system intrusions while using it.

Hi Steve_Li1688,

Your iMac only supports up to macOS Catalina. If you don't have Catalina, backup data and upgrade: How to get old versions of macOS - Apple Support.

Cheers,

Jack