With mapped invoices, they are automatically created and linked to the default nominal code for that supplier. You can however amend it as @George_H mentions by using the ‘Modify’ option, as you can normally. But with a mapped invoice, you will first need to break the mapping for that invoice.
This topic relates to deleting an invoice, but the concept is the same to modify it. Bear in mind that if the mapping is broke, any changes won’t feed through from your supplier.