It's vital that all your transactions be entered correctly so that your books can be correctly balanced. QuickBooks Online is a great tool able to help you create and send invoices to your customers all while entering them in your books. I can provide some information about editing transactions.
In order to modify an invoice for a customer, navigate to the Sales tab, then Invoices. Click on the invoice you'd like to edit in order to be able to modify it.
If this has to do with a bill to be paid to a supplier, you can edit it by going to the Expenses tab, then Suppliers. Click on the supplier's name in order to view all their transactions, then click on the invoice in question in order to modify it.
If you have any other questions, feel free to reach out here.