Start by switching to the local branch which you want to rename:
git checkout <old_name>
Rename the local branch by typing:
git branch -m <new_name>
If you’ve already pushed the <old_name> branch to the remote repository delete the <old_name> remote branch:
git push origin --delete <old_name>
Finally push the <new_name> local branch and reset the upstream branch:
git push origin -u <new_name>