What you need to know
You have uploaded a file, but it is the wrong one? As long as you have not clicked final submission, you can delete it again. This is described in step 1.
You have already clicked on Final Submission? Read step 2.
What you have to do
You have uploaded the wrong file but have not yet submitted it with "Final submission"?
Delete the wrong file by clicking the trash icon behind the file.
Then upload the correct file(s) and click on "Final submission".
The file has been submitted.
You have already submitted an incorrect file to your lecturer and have clicked on "Final submission"?
Contact your lecturer.
She/he can reopen the submission box for you again.