In the invoice mailed to the contact, there will be a link for the user to make the payment online. Once the payment is done successfully, the status of the service appointment changes to Paid. If the payment is done offline (e.g. cash), then you can explicitly change the status of the service appointment as explained below.
Select the Work Orders module from the Work Order Management menu and click the work order whose invoices you want to record the payment for.
Select the Invoices tab and click on the Invoice Number to open the invoice.
Click Record Payment.
Click Record Payment in the confirmation message. The status of the invoice will change to Paid.