Installing macOS 10.15 Catalina rather than macOS 11 Big Sur is described here: which is essentially using a hidden app store link to download the macOS 10.15 Catalina and proceeding to install as you normally would

Tips for upgrading your macOS
  • The app store download starts the process, download the app prior to planning to update as it can take time to download
  • The update process takes up to 30-90 mins during which you can't use your machine.
  • I suggest planning to do it on a Friday night or similar, and then check on it over the course of the weekend avoiding doing it before deadlines where your computer is needed
  • Plan to spend 5-10 mins post update to go though the post-update settings screens
  • While it is not common, issues with updates do happen that could make your computer unusable
  • We're here to help with any problems you might run into - just let us know