Sorry, unfortunately, at the current moment, we cannot change your username. Hence, we stress the importance of choosing your username wisely. Too many things are linked to it - billing, analytics and more stuff like that and we cannot simply alter this data. But we're working on implementing this feature.
As for now, you can delete the old account and create a new one with the correct username. Please note that in this case, the only way to restore your purchases and favorites is to contact our Support Team before deleting the old account.