These commands should let you rename the default branch in a git repo

gistfile1.txt

# On your local clone, make sure that your code is up to date with the remote (called `origin`)
git pull origin master

# Then rename your local `master` branch to `main`
git branch -m master main

# Then push your `main` branch back to `origin`, using a command that will make a new branch called `main` on the remote.
git push origin HEAD

# After this, you can:
# - Edit any existing pull requests to point to `main` instead of `master`
# - Change the "default" branch to be `main` (this applies to GitHub and GitLab remotes)
# - Delete the `master` branch if desired (or leave it, in case you run into problems)

# You can also repeat the above commands for other remotes, if applicable.