Deleting Big Sur Files

Hello,

I had decided to upgrade my Macbook Air 2018 to Big Sur. I had around 21 gigabytes of data at this time. Once the files were down downloading, I had to use the installer and pick a drive, but it said I need an additional 26 gigabytes to finish the download. Right now I have 9GB. Since I can't delete any files, I want to delete the Big Sur files as they take up 12GB that I want to keep, since I'm not going to upgrade. How can I do this?