Change ownership of iMac

I recently got a new Mac and my husband at last relinquished his old Windows PC and inherited my mac. We've more or less got it set up for him now. He has iPad and phone so has already an Apple ID.

So he has now updated the mac to Catalina and there are updates. But to enable the updates it wants MY Apple ID.

Obviously this is not good. So can anyone tell us how can we change this please.