In order to submit an assignment, click on the assignment, which looks like this:

Click on the 'browse' button - this will take you to the files stored on your PC or datastick. Choose the document that you are submitting, this will appear as a link in the 'space' to the left of the 'browse' button. Click on 'upload this file' and your assignment is then submitted.
If you want to re-submit your assignment, you must first make the changes to 'your' copy and then follow the process again, the previous document will be replaced with the new one.
Please make sure that you submit the assignment by the due date, as this makes it easier for the tutor to do the marking and provide feedback to you and the other participants.