Hi Ruth24, 

If you've voided a paid invoice, then there will be an unapplied Payment transaction in that customer's list of transactions. If you want to apply that payment to an invoice, you can click on that Payment in the transaction list, and it will show you any outstanding transactions that the payment could be applied to - all you need to do is tick the invoice that you want to apply the payment to and then click Save and Close. 

Feel free to reply in this thread if you have any other questions!

-Lucas