Locate the invoice you want to cancel, you can do this by opening the client record > selecting the Financial tab > select the invoices tab.
By selecting Invoicing from the top navigation bar > selectinig the Invoiced tab and then selecting either Invoices (if you want to search by issue) or Invoices by Date (if you want to search by the invoice date). Use the search box to refine your search.
Click on the green tick in the Paid column to open the payment box.
Click on the red minus sign and click save
Now when you view the invoice in the listing a red X will appear in the Paid column