The ability to edit or update a stock sale is dependant on what state it is in.   Draft stock sales can be edited directly, If the stock sale is Approved, you will need to Revert to Draft before you can edit.

You can edit a draft stock sale by clicking the Edit button.

Edit or update the stock sale's details and/or product line items as required and click Save to apply changes.

Approve the stock sale once saved to complete, invoice or dispatch as required.