To resend an invoice go to the Accounting tab -> Invoices. Click on the row of the invoice you would like to resend. The invoice will appear. In the sidebar on the right click the "Send Invoice" button. Some options will appear allowing you to choose the type of attachment and whether you want to send a copy to your company email address. After selecting your options click "Send". The invoice will be emailed to the customer's email account.
Note: The default view for the invoice table lists unpaid invoices. If you want to resend a paid invoice you must first select "Paid" invoices in the drop-down menu in the top right corner above the table.