Summarize this article in one sentence.
If you're not automatically logged in, click Log In in the upper-right corner of the window and enter your credentials.  It must be under the "Pending" section of the Activity tab. If the payment you want to cancel is in the “Completed” section of the Activity tab, the payee has already received the payment, and it cannot be canceled or refunded. To get your money back you must request a refund or contact the Resolution Center. At the bottom of the transaction dialog box, you'll see a message that "[Username] hasn't accepted yet" next to a Cancel link.   Your payment has been canceled.

Summary:
Go to https://www.paypal.com in a web browser. Click Activity at the top of the window. Click on the payment you want to cancel. Verify that the payment is unclaimed. Click Cancel. Click Cancel Payment. Click Yes to confirm.