I would like to update my imac

We can provide more specific advice if we know more about your computer model, age, and specifications. To get more information about your computer, go to the Apple in the upper left corner of any window, then "About This Mac", then "More Info..." Copy and paste the information here, but omit the serial number and Hardware UUID (if present).

"Click on this link to read "How to get old versions of macOS" - https://support.apple.com/HT211683

- This article also has links to system requirements for each OS version which you should check against your exact computer model. Also make sure you have more RAM than what is noted as "minimum" for any particular system or your computer may run slowly.

- You may find you need to use Safari to get the download links to open an active page in the Mac App Store application.

Back up your data in case something goes wrong --> Back up your Mac with Time Machine - https://support.apple.com/HT201250

or use a cloning tool.

Check if your old apps will still work with with the OS you want to install - https://roaringapps.com/apps This is particularly critical if you are upgrading to Catalina or newer.

You can use anything later than what you have now, including the latest, Big Sur. Review the first link I provided and decide which one seems most appropriate to you. One factor may be if you have a lot of old 32 bit software that Catalina and later did not support. Realize that eventually this will have to be replaced but if you're not ready to replace it all right now you could go for Mojave and work on getting replacements slowly.