Simply, tap on "Login or create personal account" button and then press the “Reset password” on the right-hand side menu:
Enter your username and if your account is connected to your email address, you will receive a link to reset your password directly to your email inbox. The link will be valid for 24 hours.
What to do if the game shows "Internet communication failed, please wait"? It means that you have entered an incorrect username that doesn't exist. Make sure that you use the correct username and try again.