The more important question is what is the exact model of your iMac? You need to make sure you can run a later version of macOS. You can get this information by clicking the Apple menu and selecting "About This Mac".
If you can run Big Sur on your iMac, then beware that Big Sur (and Catalina) no longer supports 32 bit apps. You will also most likely need to update your third party apps if you haven't done so yet (especially your 32 bit apps). If your third party 32 bit apps don't have 64 bit versions, then uninstal those apps before upgrading.
https://support.apple.com/en-us/HT208436
I've been seeing a lot more issues being reported with Big Sur (and Catalina) than any older versions of macOS. Definitely make sure your third party apps are compatible with Big Sur.
Make sure to have a good backup before you attempt to upgrade macOS. I also recommend creating a bootable macOS USB installer as well so you have more options in case something goes wrong.