It is possible to edit an existing bids amount or quantity, but this should be down carefully. Changing a bid in this way will not cause any of the bidding rules to be rechecked, or the quantity available for the item to be validated.
To edit a bid you must first select Advanced Mode
On the Activity tab of the Item Details page for the item you will now see an edit icon next to the bid:
To edit the bid click on the pencil icon for it.
On the bid details page you can modify the Quantity and the Amount
No calculations are done on this page: if you change the quantity in the above screenshot to 2 then the amount will remain at $80 unless also manually changed.