Unfortunately, to delete your document you must delete your video Submission as well. To do so, visit the Challenge stage of the Exercise, scroll to the bottom and select Delete Submission.
If you want to save your video Submission to re-submit it with the new document, follow these steps:
Visit the Challenge stage of the Exercise. Under your video Submission select View Submission details.
In the top right corner of your video Submission page, select the blue Download video link*
Delete your video by selecting Delete Submission on the video Submission page
Return to the Challenge stage to re-upload your downloaded video and new document.
*If you're using a Mac, you will be taken to a page with your video. Right-click on the video and select Save video as to save the video file to re-upload.