@Bob72: Are you currently inside the region where the device was intended to be used? If you haven't had any luck over the air via Settings > Software update > Download and install, then you can try Smart Switch on your home computer.
To begin with, please follow the link below, and click on 'How Do I Get Smart Switch On My PC or Mac' to download the relevant version
https://www.samsung.com/uk/support/mobile-devices/how-do-i-backup-my-smartphone-to-my-pc-or-mac/
Once installed, open Smart Switch, connect your S21+ using the USB cable that came packaged with the device, and look for an update prompt on the landing page, next to your device details.
