So, you realize after you entered a payment from a customer that you entered it on the wrong order. Your first thought might be to just delete the payment from the wrong order and enter a new payment for the correct order. While there is nothing wrong with doing that, we do have another solution:
Open the Payment record that needs to be modified
Click on the Split Payment button at the bottom of the screen
This will open the list of orders for the customer
Uncheck the box next to the invoice where the payment was initially applied (which in the example below is invoice 6001)
Check the box next to the order where the payment should be applied (which in the example below is Work Order 5165)