Hi George,

If you want to open DWG files in 2021 version of AutoCAD from Finder, you need:

- open Finder window, navigate to folder with DWG files

- select one of them, right-click and select "Get info" from context menu

- in Info window go to Open with section and select AutoCAD 2021 from drop-down list

- now pick on "Change all..." button and confirm with "Continue" button:

You can safely remove 2018 version from your Mac, all DWG files will open in AutoCAD 2021.