The quantities and the prices in a Approved or closed Document/Invoice/Bill are not editable. You will need to cancel the issued documents and create a new one where you will be able to edit the data.
1. Open the document you wish to change and Cancel it under the More men
2. Clone the Document Data to a new Document under the More menu
3. You can find the new Document that has been created in the History tab, no number has been assigned to it yet
4. Click on the "--" of the document, the new document will be in the Pending state and will be completely editable.
5. If the document has been marked as Closed, you can no longer modify it or cancel it unless you re-open it.
Or login to your account.