The answer to this question will depend on whether your submissions has been Finalized or not; in both cases the project must still be in the Open stage (i.e., accepting submissions) in order for a vendor to change their submission.
Submission Not Finalized
If your submission hasn't been finalized simply click the trashcan icon in the file that you need to replace:
You can then upload the correct file in the appropriate upload slot.
Submission is Finalized
First make sure you are logged in using your email and password. Once logged in, click on the Submissions link in the black ribbon on the top of the page.
Once you're on your Submissions Portal page make sure you're viewing the appropriate project (if you have more than one project in the system).
Next, simply scroll down to the bottom of the Submission Portal 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 using the instructions above.