Can I change/adjust an existing offer?

No, an existing offer cannot be adjusted. However, as a seller you can cancel the offer, even if there is already a bid made on the offer.

As a seller to cancel an offer, go to dashboard and the account related to the eyeventure. (as shown on the photo below) With every offer, under action, there is a trash-can button. By clicking on it, you will cancel the order and is no longer visible in the bank statement.

There will be a message posted in the community under the eyeventure transactions so everybody involved knows that the order was canceled.