Follow instructions at https://docs.github.com/en/github/administering-a-repository/renaming-a-branch.
The short instructions are to click on the branches label, and click on the edit icon to rename the branch.
All individuals will need to run the following commands at their git repository. This will need to be done for each repository that is being renamed.
git branch -m master main
git fetch origin
git branch -u origin/main main