How do I fix endless spinning circle while updating my iMac?

Have you installed and run any "cleaning", "optimizing", "speed-up", anti-virus or VPN apps on your Mac?

Try booting into Safe Mode according to How to use safe mode on your Mac and install from there.

Note:  if you have a wireless keyboard with rechargeable batteries connect it with its charging cable before booting into Safe Mode.  This makes it act as a wired keyboard as will assure a successful boot into Safe Mode.

Unwinrich wrote:

I’ve just found out my Mac has updated the OS since I purchased it in 2020. Some features now don’t work and I get the message update OS.

when I try to search for updates all I get is a spinning circle that never ends, no message whatsoever.

This sounds like a software corruption issue; there are numerous ways to try and fix this including running First Aid on Disk Utilities.

I have auto update selected, it never does anything,

Remember that an update to macOS 11 only takes you to and updated version of macOS 11.x. You need to upgrade (not update) to macOS 12, 13, 14, etc. or whatever your Mac is capable of running.

But run First Aid first!

Which exact model of Mac do you have? We can tell you what the latest macOS is for it.

What was the result of Disk First Aid?

Your old iMac (Retina 4K 2019) should be able to run macOS 14 Sonoma which can be downloaded from

macappstores://apps.apple.com/gb/app/macos-sonoma/id6450717509?mt=12

You can only use this link using Safari.