To change the username in "Minecraft," access the Mojang account, and click the Change option that appears next to the profile name. Choose an available username, and change the username to the new one. Usernames in "Minecraft" can be changed in this manner at no charge.
A user who signs in into his Mojang account using his email ID can change his "Minecraft" username by accessing the Mojang account log-in page. A user who signs in with his username must migrate to a Mojang account to be able to change his username.
The username, once changed, cannot be altered for another 30 days. First-time usernames must also wait for a certain time period before being changed. If a particular username is not available, then the old name cannot be changed to that one.
Additionally, the old username is retained for 37 days, which gives users seven extra days to revert to the old username if they desire to do so. Also, once a username is changed, the old one can be seen by others who are looking for a username of their liking.
Username changes are completely supported until the "Minecraft" 1.7.10 version. In game versions older than this, the changed username is treated as a new one. Mojang Support does not help users change their usernames.