I need to edit the quantity of an item on an invoice and resend it to my customer - how do I do this?

Welcome to the Community, @mel29.

You can edit the quantity of an item on an invoice by opening the invoice, edit the QTY field and click Save and send. 

Here's a helpful article about recording invoice payments in QuickBooks Online: https://quickbooks.intuit.com/learn-support/en-us/undeposited-funds/record-invoice-payments-in-quickbooks-online/00/186418. This article has info on how to record single invoice payment, Group multiple payments together into a single record, Record partial payments, and more. This may be an article for QBO US, but works similar to the international version. 

If you have other questions, feel free to drop a comment. I'd be glad to help.