There are various reasons why one might want to rename the default branch of a Git repo from master to main. Here’s a quick reminder to myself of how.
On GitHub
This is easy. From the main page for your project, click on the 2 branches link above the file list, and then click the pencil icon next to the branch in question.
As well as renaming the branch it will change existing pull requests to target the new name.
On GitLab
There is no automated mechanism (as of 2022-09): the suggested approach is to rename it locally and push the new branch:
git checkout master
git branch -m master main
git push -u origin main
You can then set it as the default branch, and protect it, via the GitLab UI. You also need to instruct merge request owners to retarget any extant merge requests.
On local checkouts
GitHub makes this easy by telling you the incantation, which I will here repeat:
git branch -m master main
git fetch origin
git branch -u origin/main main
git remote set-head origin -a
If, like me, you have a second checkout on your web server, then you need to run the incantation there as well.
The GitLab recipe substitutes the following for the fourth line above:
git symbolic-ref refs/remotes/origin/HEAD refs/remotes/origin/main
I think this has the same effect (but avoids a needless network transaction).