From 1.8.0 version of the game, you can edit your nickname if you're playing as guest. Tap the main menu (3 horizontal lines) and then 'Open my Profile' (or just your avatar).

Just at the right of your guest number, you have an icon to edit your nickname. Tap and follow the instructions.

Please note that we save all the nicknames you validate in our database, so we will be able to track if one of your nicknames go against the rules. Everyone can also report a nickname, so be sure your wont be controversial.

You can edit your nickname as often as you like. Your nickname has to be between 2 and 12 characters long to be valid.