If i want to it says my name has invalid letters?

please answer soon!

Well, you may have been trying here, which looks right but is actually where you change your username. (Note that it gives you the "invalid characters" message even when what it means is "That name is already in use"

To just change your password, not your name, use this.

It is a good idea to logout, delete all cookies from secure.gamesas.com, then log in again before making any big account changes. Try that if it gives you trouble.