Beware, this action is VERY dangerous and might ruin some of your running applications, I tried it first on a virtual machine and then my actual machine and it messed up 2 or 3 apps only, I was lucky as these were easy to fix, here we go (Read the instructions below carefully):

usermod -l newname -m -d /home/newname oldname

Please note that you MUST:

1. be logged out from the user that you will change the “username” for 
2. and make sure all that user’s applications are closed.

Thanks go to prshah for his post here