If you have named a branch incorrectly AND pushed this to the remote repository follow these steps before any other developers get a chance to jump on you and give you shit for not correctly following naming conventions.
1. Rename your local branch.
If you are on the branch you want to rename:
git branch -m new-name
If you are on a different branch:
git branch -m old-name new-name
2. Delete the old-name remote branch and push the new-name local branch.
git push origin :old-name new-name
3. Reset the upstream branch for the new-name local branch.
Switch to the branch and then:
git push origin -u new-name