A 'void' is a technical term for reversing a payment on an account. One reason you may want to void a payment is if the payment was captured incorrectly, or with the incorrect payment method.
From the account, make sure that the account is open. If it is closed, you may need to re-open it:
Once the account is open, click on 'Void Payment'. This will allow you to input the amount you wish to void, and the payment method you want to void. Be sure to make the void date the same date as the original transaction:
Doing this will create a new line item on the account which will balance out the incorrect payment. You can now capture the correct payment for the account.