Please - clarify if you would. The Profile with your ID indicated iMac 21.5″, macOS 10.15. Are we mixing update with UpGrade ? An upDate is always from within the same macOS and upGrade is moving from Catalina to Big Sur.
If this is the upGrade - suggestion. Shutdown Router. Shutdown Computer and disconnect all external drives. Restart Router. Restart computer in Safe Mode - Shift - Immediately at startup. It does a Repair Disk, clears cache files and Only loads Apple Software and Fonts. So it will load very slowly - Normal. Once logged in the computer will look and feel differently - Normal. Open the Apple Apps Store and look for Big Sur and commence the download. Understand - this will be very big as the Full Installer is 12.24 GB in size. It will pause at different stages of the download - normal. It may even appear to pause for log periods of time - the countdown clock may say 15 minutes but in Real Time could be 45 minutes, This may occur several time is the download process. Once it is 100% done it will attempt to launch - Stop it and do not allow it to proceed. Suggestion again, look in Application Folder for New Application called Install Big Sur - make a copy to reliable External Drive for future usage - like USB Installer. Then and only then launch the Installer and follow the prompts.