Assuming you're currently on the branch you want to rename:
git branch -m newname
This is documented in the manual for git-branch, which you can view using
man git-branch
or
git help branch
Specifically, the command is
git branch (-m | -M) [<oldbranch>] <newbranch>
where the parameters are:
<oldbranch>
The name of an existing branch to rename.
<newbranch>
The new name for an existing branch. The same restrictions as for <branchname> apply.
<oldbranch> is optional, if you want to rename the current branch.