If you have entered an offer on the listing activity page, it is not possible to update the amount or terms. You can add a new offer to the person whom the incorrect offer was entered against.
When creating a vendor report, if you are adding the offers page and don't want to include the incorrect offer, you have the ability to not select the incorrect offer and only select the correct one. You also have the ability to update the amount of offers made on the campaign overview widget to ensure that this one is not showing.