I saw some people wandering how to do this. So I did a bit of research on the matter.
Changing usernames
We do not yet support changing or editing usernames, though this is planned for the future. We do not have a set release date for this feature. Make sure that you choose wisely, as your username will be used for your multiplayer persona and is visible to others.
As of Minecraft version 1.7, servers will be equipped to use players’ unique UUIDs. This means that when a player is banned from a server, they will remain banned even if their username changes. Additional changes will be made to servers and accounts before username changes are available.
Due to the extra features in Mojang accounts, if you would like to change your username in the future, your Minecraft username will need to be on a Mojang account. If you currently log into the game with your username, you can migrate to a Mojang account at account.mojang.com/migrate.