How do i change my new apple id on my iMac? Phone and iPad is fine but iMac is still showing old id after email change, how do I rectify this?

How do i get my iMac to recognise my new apple id? When I try to access iCloud on my iMac my old id is automatically populated on the pop up screen. My phone and iPad recognise my new id after email change so I can access everything on both these devices but iMac is still showing old id (old email address) , how do I rectify this?