Upgrade to Big Sur
I am trying to upgrade from Catalina to Big Sur on my desktop Mac computer. But when I select "Software Update" in System Preferences, it does not show the Big Sur icon, and says my OS is up to date.
How do I upgrade?
iMac Pro
You need to have a 2014 or later iMac to upgrade to BigSur.
see > https://https://support.apple.com/en-us/HT20163
What year model is your iMac?