Where do I find a course that I need to resubmit?

Go to your In Progress tab in the blue banner at the top. Click on the Feedback button on the left sidebar and then click on the course card.

When the course page opens, you will see the feedback that was given by the reviewer, and what you originally submitted whether it was a link, image, or text response. Click Resume Challenge and the challenge window will open back up for you to resubmit.