To delete a remote branch, you can’t use the git branch command. Instead, use the git push command with --delete flag, followed by the name of the branch you want to delete. You also need to specify the remote name (origin in this case) after git push.
git branch -a
Will give you an overview of all branches of all remotes ...
git push origin --delete testbranch
Will delete the branch 'testbranch' in the remote repository named origin