If you finalized a submission and would like to change one of the files uploaded you must first "un-submit" your submission. The following short tutorial will take you through the steps of un-submitting:
First, make sure you are logged in using your email and password (click on the Login /Register link on the top right).
Once logged in, click on the Submissions link in the black ribbon on the top of the page.
Once you're on your Submissions page click View Receipt for the project you wish to revise the submission for.
Once on the receipt page, simply scroll down to the bottom of the page and click the link to un-submit your submission.
After you un-submit you will have the opportunity to re-upload any files again, or upload additional files (if required). Remember by uploading a file a slot with an existing file you will overwrite the existing the file.
After revising your submission, make sure you Submit and Finalize your submission (you will receive a new email confirmation receipt with a new confirmation number).