In order to void the invoices on the iPad, you need to proceed with the following steps:
1. Go to the "Billing" tab.
2. Select the invoice and click on "View Details".
3. Click on "Void" at the top.
4. It gives a pop up asking you to confirm the voiding process.
5. Then, hit "Save" at the top right corner.
6. The invoice gets voided.