If you want to rename the branch you are currently working on, hit below command:
git branch -m <newBranchName>
If the branch you want to rename is different from your current branch, then hit below command
git branch -m <oldBranchName> <newBranchName>
To check your current branch, hit below command. You can see the current branch highlighted followed by an asterisk.
The same question has also been asked here as well @ StackOverflow.