Log in to My Y!mobile and tap "Change Address."
(See How to Log In for instructions on how to log in.)
You can check and change your contract address and billing address.
To complete the procedure, please prepare your SoftBank ID (Y!mobile mobile phone number or alphanumeric ID) and My Y!mobile login password.
If you are using this service for the first time, please prepare your Y!mobile mobile phone number and PIN (4 digits).