Where can

virlecia wrote:

how can I update my mac from Catalina to Big Sur?

Update is going from Catalina 10.15.6 to Catalina 10.15.7

UpGrade is going from Catalina macOS 10.15 to Big Sur macOS 11

he 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.

macOS Big Sur 11 hardware requirements

Make Time Machine Backup  is very useful  and Before attempting the upGrade

Here is Apple's guide to downloading older versions of macOS: How to download macOS - Apple Support

Not that you will need to use Safari to do this.