The -m flag can be used with git branch to move/rename an existing branch. If you are already on the branch that you want to rename, all you need to do is provide the new branch name.

$ git branch -m <new-branch-name>

If you want to rename a branch other than the one you are currently on, you must specify both the existing (old) branch name and the new branch name.

$ git branch -m <old-branch-name> <new-branch-name>

h/t Dillon Hafer