If you used your existing account with Premium Student to subscribe to Premium Family, your subscription should switch automatically.
Keep in mind that you could have been charged for the price difference while changing your plan. It's explained here.
However, if you created a new account or used a different one to subscribe to Family, you can log in on your account page (Make sure to use the credential for the account with the Student subscription) and cancel there.
Note: There are a few ways to sign in, e.g. with your email, phone number, Facebook, or Apple.