software update

The Age of this computer will Govern what newer version of macOS it will Qualify to install. Use About this Mac from the Apple Icon on Desktop - the required Information will be there.

With the above information in-hand - follow this How to get old versions of macOS and verify what version this computer Qualifies to install.  For Best results use Safari to commence the download as Others may not work.

Safari will open a redirect and open the Apple Store where the download can commence.

Notation - there is only one FULL Version of Big Sur offered by Apple which is version 11.6.4 . There is no version 11.3 Available

macOS Big Sur 11 hardware requirements

Empty Space Requirements specific to the Big Sur. Apple states 35.6 GB Empty Space but suggest 50 GB

Tips Big Sir Upgrade Released Nov 2020

Make Time Machine Backup  is very useful and can be used to Revert to Previous Working macOS

If your Mac meets the hardware requirements for Big Sur, follow the steps in this article to install macOS 11 on your iMac.

How to get old versions of macOS - Apple Support