You'll be able to change your username from the "Settings" menu. To do so, select the cog icon on the upper right corner of the game's main screen.
When accessing the menu, tap on your username or profile icon. Then, your account information will unfold, and there you'll be able to edit your username, tapping on it.
Keep in mind that your username must be unique and that the use of capital letters will not be accepted. Therefore, remember that if you receive an error message, it is possible that the name you have selected is being used by another user. In the event that this happens, we recommend entering a new alpha-numeric combination together with your username.