It is possible to edit an existing bid's amount or quantity although this should be done 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.
On the Activity tab of the Item Details page for the item click on the menu icon next to the bid to edit.
Select edit to go to the bid details page. Here 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. You will need to manually change the amount to reflect the new quantity as needed.