From a Computer:
Visit https://account.sonyentertainmentnetwork.com, sign in to PSN™, and then go to the Subscriptions Management section. After selecting the subscription you want to cancel, cancel the automatic subscription renewal. Call Consumer Support at 877-971-7669. 
You need to log in your Realm once every 60 days for the Realm to remain active. PlayStation®Plus membership is required to play on PlayStation® (sold separately). If your subscription is inactive for more than 18 months, your Realms data will be deleted from the server. ISP fees may apply. Realms discounts cannot be combined with other offers and are not redeemable for cash; Requirements, features, online services, or subscription subject to change and/or retirement. Subject to the Microsoft Services Agreement (microsoft.com/msa), Minecraft EULA (www.minecraft.net/eula), and platform-specific Minecraft terms (available via the Terms & Conditions button above).
