How can I change my Minecraft username?
Simply said, this is not possible. As stated on the official Mojang site:
Changing usernames
We do not currently support changing, editing, or correcting usernames, though this may be a feature in the future. Make sure that you choose wisely; it will be used for your multiplayer persona and is visible to others.
However, Mojang is working on a way to make this possible in the future using the new Mojang accounts system. For now though, you can't change it.