You can change your username by tapping on the icon on the upper left of the Home screen to go to the User Data screen, and tapping the pencil icon next to the username.
You can use up to 10 full-width characters.
If the same username exists in the server, it cannot be used.
You can change your username for free for the first time.
You would need Gems if you wish to change it for the second time onwards.
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