To edit your scheduled broadcast, simply click on "Scheduled streams" and select the correct broadcast from the dropdown menu:
Afterwards, stream settings will appear on the left side of the studio. From here, you may edit or change any broadcast details. Bear in mind that you can only change the time/date, cover image (in .jpg or .png not larger than 3 MB), and stream title/description.
Lastly, don't forget to hit Save to apply any new changes.