You can rename a branch both locally and remotely with the following steps:

I. Rename Locally

git branch -m dev-branch main-dev

This renames the branch you're currently on. If you're not on dev-branch, use:

git branch -m dev-branch main-dev

II. Push the Renamed Branch

git push origin main-dev

Then delete the old branch from the remote:

git push origin --delete dev-branch

III. Reset the Upstream Tracking

git push --set-upstream origin main-dev

IV. Update Default Branch on GitHub (Optional)
If this is a default branch, go to your repository’s Settings > Branches and set main-dev as the new default. Then delete the old one.