On the Details page of the payment you need to edit, scroll down to the bottom of the page and click the Edit Payment button

In the Payment From section, click the Change Individual (or Change Organization, depending on the original donor type) button 

In the Payment Now From section search for the correct donor 

Click the Select button under the correct donor 

Verify the changes 

Scroll down and click Save & I'm Done

The Payment Details screen should now reflect the new donor