You can cancel a payment you've already made
if the receiver hasn't yet actively claimed it.
If they have, you'll need to
dispute the payment or request a refund.
If they haven't claimed the money,
first log in to your PayPal account
at Paypal.com.
From the top, click on the "Activity" tab.
You'll see all your recent transactions listed.
Find the transaction you want to cancel
and then click "Cancel" here.
A confirmation pop-up window will appear.
Click on "Cancel Payment" to confirm.
The money will now appear in your PayPal account
within a few days.