Select Your “MainUserName”
Select Account Settings.
Select Add Names.
You’ll arrive at a website that looks like this. Your MainUserName is in the left slot. The two right slots are for equipping alternate user names.
Click on Add Name.
You can create a new user name and select ADD. or you can click where it appears red in the image below and add an already existing user name.
You can “unequip” a user name as well to make room for a new one. This can be undone at any time by following all of the instructions presented here.
Click the existing user name.
Select Remove User. You have now freed up a slot for another alternate user name.