I've implemented a new command magit-update-default-branch, which might come in handy, considering that I just renamed the default branches of all my repositories. You can just press "M b" and confirm. That's all there is to it now.

If you would like to rename the default branches in your own repositories, you can do so using the new forge-rename-default-branch on "M b r".

(The latter currently only works for GitHub and the binding for the former changes to "M b u", once Forge is loaded.)