Try to disable the branch protection in the settings/repository page. Check the sections Default Branch and Protected Branches
After that:
Checkout the branch locally.
git checkout branch
Rename it locally
git checkout -b branch_old
delete remote branch
git push --delete origin branch
push the locally renamed branch to remote.
git push --set-upstream origin branch_old
How do I delete a local branch in Git?
Git makes managing branches really easy - and deleting local branches is no exception:
git branch -d <local-branch>
In some cases, Git might refuse to delete your local branch: when it contains commits that haven't been merged into any other local branches or pushed to a remote repository. This is a very sensible rule that protects you from inadvertently losing commit data.
If you want to delete such a branch nonetheless (e.g. because you've programmed yourself into a dead end and produced commits that aren't worth keeping) you can do so with the “-D” flag:
git branch -D <local-branch>