How to override unit price that default from Purchase Order?

It is mentioned in Oracle Payables Student Guide , under the Topic Entering Purchase Order and Receipt Matched Invoices

" Price corrections are very different from overriding the unit price when matching an invoice to a purchase order. When you are entering an invoice and matching to a purchase order, you can override the unit price that default from Purchase Order so it is the same as the unit price on your invoice. You use price corrections only after the initial match.

My question is how to perform this step ?

Thanks and regards

Mateen Khan