To reset your password, simply launch your Sonic App (but please make sure that first you're signed out of it). On the login page, tap "Forgot password?" and you will be directed to a new page to enter your email address.
Once you have done this, please check your email inbox and follow the instructions in the email. If you do not receive the email after a few minutes, please check your spam folder.
NB: Once you've changed your password, the link will expire.