If you used a fake email address to create your sync account, you have no possibility to retrieve the password or set a new one. Please create a new sync account, either using a valid email address or a fake one. Retake the steps to sync your lists, same as if you created a new account. Find the required steps here.

If you used a valid email address to create your sync account, you are in luck! You can request a password reset email. Open the Synchronization tab (three dots menu). Tap the three dots again to log out. Open the Synchronization tab again and right under the Log in button there is a blue text that says Reset password. Tap it, and then input the email address you used to create your account. Tap Reset password. An email will be sent to the specified address and you just have to follow the instructions in the email to reset your password.