• Tap the Invoices tab.
  
• Click the Unpaid or Overdue status from the drop-down list.
  
• Tap/Select the invoice : Lands to the Invoice Details page.
  
• Tap the action button from the Android device.
  
• Tap the option Payments.
• Tap icon
• Update the payment details such as : Received Date, Amount, Mode, Notes.
• Tap on the save icon.
  

You could check-mark (enable) the option send a thank you note. to thank your customer for the payment.