I issued an invoice which has now changed. Do I edit it or is it best to delete and issue a complete new one

I can help you with that, gundrylisa.

Editing or deleting an invoice will make a difference in the way your books are organised in QuickBooks Online. 

If your going to delete the invoice, it will be permanently removed from your company file. However, editing an invoice shows new values in your accounts. 

To edit an invoice:

1. Go to Sales menu and select Invoices.
2. Locate the Invoice.
3. Click the drop-down arrow under Action.
4. Select View/Edit.
5. Make the necessary changes.
6. Click Save and close.

To delete or void an invoice:

1. Find and open the invoice you want to delete.
2. In the Invoice window, click More at the bottom.
3. Choose either Delete or Void, then click Yes to confirm.

I'm adding this article for more guidance: Void or delete transactions in QuickBooks Online.

Should you need any assistance managing your QuickBooks transactions, I'm available here to help any time!