As an advertiser, you will be able to cancel the pick-up through the ad's menu "Cancel pick-up" after the time slot has expired. The helper will be informed and your ad will open up so that you can get help from someone else.
As a helper, you may abort a pick-up at any time through the ad's menu "Cancel pick-up" if for some reason you can't fulfil the pick-up. You will then be asked why you want to cancel the pick-up. Unfair reasons will affect your rating which could make it more difficult for you to be selected for pick-ups in the future.
The easiest way to find the ad is to open the app and click on "Activities" in the bottom dark area.