This can be done under Profile and settings:
Login to your PayPal Account.
Click on "Profile and settings".
Click on "My Money".
Look for: "My Preapproved Payments".
You should see: "Manage the automatic payments, subscriptions, and installment plan payments I send to merchants".
Click on "Update".
Next, you will see all active and cancelled billing agreements/subscriptions listed on the screen.
Click on the service-name for further action.
You should see on next page terms of billing agreement/subscription, how much money you have authorized service provider to deduct from your account and few other details. Most important is link to Cancel agreement, in case if it is active.
Note, when you click on Cancel, you will be prompted to confirm it once again. Click "Yes".
As a last option, if you're stuck - suggest that you contact customer support - they can walk you through the steps. You can contact them via email, phone or through Twitter @AskPayPal or via Private Message on their Facebook page at https://www.facebook.com/PayPal
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