'settings' >> 'Code and Automation' >> 'Branches' >> 'Default Branch'

On GitHub.com, navigate to the main page of the repository.
Under your repository name, click "Settings".
In the "Code and automation" section of the sidebar, click "Branches".
Under "Default branch", to the right of the default branch name, click.
Use the drop-down, then select a branch name.

source: 'https://docs.github.com/en/repositories/configuring-branches-and-merges-in-your-repository/managing-branches-in-your-repository/changing-the-default-branch#changing-the-default-branch'

git branch -m deploy prod
git fetch origin
git branch -u origin/prod prod
git remote set-head origin -a