How to remove the paid stamp from an invoice?

You need to unapply the payment from the invoice.  I would go to the customer's account and double click on the payment from your customer.  This should take you to the Receive Payment screen that you used when you originally entered the payment.  Remove the check from the invoices you don't want to receive payment.  Or maybe you want to receive partial payment for this particular invoice.  In that case, apply only partial payment from your customer to that invoice.  As long as the invoice has not been paid in full, the "Paid" stamp will not be on that invoice.