I have made a user in Mint 16 but want to change its first and last name without deleting it and starting from scratch. I know how to do it with a GUI but I want to know the terminal command alternative. I've checked the man pages of usermod but couldn't figure out how in sh.
There are a couple of ways, usermod is a good tool that comes to mind (I have trouble with it at times).
chfn this will generally give you a lot of options to change (phone number, office location, and other stuff).
# chfn username should open your favorite editor for you to change things with.
There are some unsafe methods, but I don't recommend them.