The system will allow the date to be amended on a Sales Invoice if you have not yet delivered it from the system. Also you should consider before making this change if it is appropriate for your internal accounts process, especially if you have exported the sales invoice to your accounts system. To change the invoice date you need to go to Invoice Edit within the Sales Invoices menu:
Make the changes to the date then click Recreate Document to recompile it. Next click View Document to ensure the changes have been applied correctly. If the invoice has been delivered from the system, the Invoice Delivered? box shown above will be checked and the Invoice Date field will be disabled. This can be overridden by removing the tick however, and the invoice will be updated ready to send/resend to your customer as appropriate.