You can edit a fast in real-time by clicking on either the start time or the target end time - or by clicking the plus or minus button to increment/decrement your fasting goal. When ending a fast, you will also have the ability to edit the start or end time, or even delete it altogether.
If you want to edit a fast that you've already saved, go to your Profile page. You will see your fasting history there in both a chart and a table. Click on any of the entries in the table and you will be able to edit or delete them to your heart's content!