I tried downloading the recent upgrade of mac OS "Big Sur," but I am getting a message "not enought free space on the selected volume to update to OS. An additional 9 GB is required. How do I upgrade the additional 9GB?

You don't add the 9GB. You have to free space on your drive.

See: How to free up storage space on your Mac - Apple Support

In most cases you will need 35.5 GBs of free space to perform the upgrade. The upgrade won't occupy that much space, but that much free space is needed to perform the upgrade.

You will need to remove or delete enough files to meet the requirement.

The files that you have control over are located in the Documents, Downloads, Pictures, Music, Movies and Applications folders.  You can use either of these two free apps, GrandPerspective  or OmniDiscSweeper, to find the largest files on your drive so you can determine if they can be deleted or moved to an external HD for storage.  

Note: you can empty the Downloads folder after the apps and/or updates that were downloaded have been installed or applied.  Many users have found a couple of Gigabytes of files in their Downloads folder which are no longer needed.