Yes. Simply head on over to your Profile and look for the "Edit Profile" (little pencil) button. From there, you'll be able to change your username, but keep in mind that you can only do so once per month.

Additionally, once you change your username, you may not be able to switch back to a previous username if it's already been taken by someone else. Therefore, it's important to choose your new username carefully and make sure it's something you'll be happy with for the foreseeable future.