From the Payment Requests page, find the Request you wish to re-send and click the 'Manage' chevron on the right of it.
You can now see the details of that Payment Request. One of the details is the original Lead or Leads you sent. Selecting it (or the appropriate one) will open it. In the screen, you'll see the 'SEND' option opening a dropdown to either re-send the email or text message.
Select which method you would prefer and this will resend the initial Payment Boarding request.