You can only set your username when you register, and it cannot be changed.
You’ll be asked for your nickname at the latest when you log in to a Fediverse service that also requires a nickname (fairmove.net, fair.tube, etc.). You can also set your nickname on fairapps.net in the top-right corner of your profile under “Personal Info.” We recommend not changing it, because otherwise your Fediverse ID will no longer match.