You cannot change your username as of right now. Administrators can will only change it under one circumstance, and that is if your username contains personal information! Your username doesn't appear to have any personal information, so you will have to wait.
There will be a feature in the future which allows you to change your username. That will not be available for at least another year.
If I had my own world
I’d build you an empire
from here to the farlands
to spread love like violence