In Reckon One, once an invoice is paid, that invoice becomes read only (e.g. locked).
There are two options you can choose from:
Option 1: Remove the invoice payment transaction (e.g. receipt), then update the invoice date & re-enter the invoice payment (receipt).
Option 2: Unlink the invoice payment transaction (e.g receipt) from the paid invoice, then update the invoice date & re-link the invoice payment (receipt) to the invoice.
To unlink the invoice payment (e.g. receipt), open the applicable paid invoice & click the "Already paid" amount which is shown in blue text in the totals section of the invoice which will then allow you to unlink the invoice payment.
To re-link the invoice payment (receipt) after you have modified the invoice date, open the applicable invoice & click "Receive payment". The "Receive payment" screen should show a credit available, click "Apply this credit" & then select the credit transaction & save.