Unfortunately, we can't change your username and/or avatar linked to your game account. You can, however, change your username and picture between your Facebook Account and your Miniclip Account.
Just go to https://www.miniclip.com/user/settings/en/ on your computer and log in with your Facebook account and select between the 2 names/pictures displayed. Make sure to login using the Facebook login option.
On the mobile version of your game, just go to Player Profile > Select the edit avatar icon > Choose your avatar > Done!
Keep in mind that if you don't have a Facebook account connected, you will have to keep the initial username you’ve used when the account was created.