On the Invoice Screen there will be a full list of invoices all sequenced according to the date timeline (from the earliest date to latest date) as per the far left column.
To see which invoices have not been paid, start by sorting them by Payment status.
PROTIP: Click on the green Payment status button in the dropdown menu to sort your invoices and it will first give you your paid invoices at the top.
Click on the same Payment status sort button again will then swap the list to show all your unpaid invoices at the top.
Once you have identified the UNPAID invoices, click on the purple RESEND button next to the invoice for it to be sent again to your client.