Please log in to your Deezer account from a PC/Mac and follow this link: http://www.deezer.com/offers/family to change your subscription to Deezer Family.
Settings>Account Management>Manage my subscription>Switch to Deezer Family
If you are a Premium subscriber and wish to upgrade to Deezer Family, you will immediately be charged for the Family subscription and the plan will start as soon as the payment goes through. Your Premium subscription will be terminated at the same time.
How to upgrade through iTunes
If you're currently subscribed through iTunes, all you need to do is log in to your Deezer account via your application on your device.
Then, go to the Manage my subscription and Change plan
If you're paying through the Apple App Store, your total charge will be slightly more due to the Apple commission charge