how to update my mac

If you're running Yosemite (10.10) now, it seems likely that your iMac cannot run Ventura – that if you can upgrade to something newer than Yosemite, and older than Ventura, you'll have to get it through this link:

How to download and install macOS - Apple Support

What model name do you see if you select About This Mac from the Apple () menu? No serial numbers, please. The model (including release year/time) will determine how far you can upgrade.

An iMac (21.5-inch, Late 2013) can be upgraded to macOS 10.15.* (Catalina), but no further.

Catalina does not run 32-bit applications, and it is no longer one of the "most recent three," so it may not be new enough to run current applications from companies like Microsoft and Adobe, either.

What software are you looking for?