Once a payment has been recorded, an invoice generally cannot be edited further.
If an invoice is in Partially Paid or Paid status because of a credit card payment processed by Stripe or PayPal, it is not possible to edit it moving forward. This is primarily due to payment record-keeping purposes for Pixieset and the payment processor.
If you wanted to invoice your client for an additional amount, consider creating a second invoice. Alternatively, if due dates have changed, you can confirm this with your client separately; rest assured that payments can still be made for invoices that are past due.
If the invoice is only Partially Paid or Paid because of a manual payment that you have manually recorded yourself though, you can edit it by deleting all the recorded manual payment(s) within the invoice so that it is set back to $0.00 in the amount paid:
When this is done, the invoice will be set back to Unpaid status, which would allow you to edit your invoice again. Once you have updated your invoice, you can re-log your client's earlier payments and re-share the invoice as required.