Big Sur is out there on the App Store.
How to upgrade to macOS Big Sur - Apple Support
Make sure your mac has at least 35.5GBs of free space.
Have your mac connected to your router via cable.
Go to Section 2 and click on Get macOS Big Sur
this will redirect you to the App Store, give it a minute or two
and open the Big Sur download page it should say Get.
Pressing Get will probably open the System Preferences> Software Update
panel and start the download.
Do not use the mac while it is downloading or any other devices that share the same
internet connection.
Have you checked that any third party apps you use in Catalina will still work in Big Sur.
Also check that any third party hardware, printers/ scanners etc have drivers available
so they will continue to function in Big Sur.