Where can the customer change their address?

The customer can change their address in the customer portal if you enabled this option in Seal Subscriptions > General Settings > Settings > Customer portal. The best way to access the customer portal is through the magic link which the customer gets into the email when they start a subscription in your shop.
Also, a thing to know here is that the customer has to change address on each subscription separately. Changing it on their account in your shop won't update the address in their subscriptions.