On occasion, a piece of creative may be rejected by an admin at the media company. Could be that it doesn't meet a particular spec or it might be deemed inappropriate for the audience and a suitable alternative will be required.
If this happens you will be notified to come back to your campaign and update the details. To do so, select "View Details" on the affected campagin:
From the details screen select the middle tab, "Creative Assets"
In the lower right, there will be some admin feedback about what is required from you.
Click the + Replace button and navigate on your file system to locate the replacement creative.
This will re-submit the order back to the admin to review and approve.