It sounds like you've currently subscribed to regular Premium through your mobile provider. In that case, in order to subscribe to Premium Family, you'll need to firstcancel your subscription, as you mentioned.
Once your account has reverted to Free, you can subscribe to Premium Family here.
Keep in mind that when your account reverts to Free, you won't lose any playlists or liked songs. However, once you subscribe to Premium Family, you'll need to make available for offline use any playlists you had downloaded previously.