You can't change your username from your XBL Gamertag to something else. Most servers don't allow usernames with spaces. I'm an idiot who chose to have a space in his XBL gamertag. In this case, there is 3 ways. 1) Not being able to play MC:W10E online 2) paying $9.99 to just remove a space from my name. I'll not do such thing as I don't like spending money because of a bug. 3) This bug gets fixed, either by allowing users to change their usernames, or removing unsupported characters in usernames while joining servers or by allowing the characters allowed in a XBL Gamertag in Minecraft Pocket Edition servers.