Assignments have a few variations on the types an instructor might use, but the most common is the type where you click on the assignment on the course page and then you will see a "Browse" button. Use that to locate the file on your computer. Select and upload it. Most of the time, the instructor will have it set to where you can re-upload a file to replace the original or upload more than one file. If the instructor has set a due date on the assignment, the browse and upload buttons may be disabled after the due date has passed. Be sure that you have selected the correct time zone in your account profile. If it is set incorrectly, you may be turning it in on time by your own wall clock but the system will think you live in a different time zone so it could be coming in late by what it thinks.
NOTE ABOUT DUE DATES:
Some classes will have due dates. If there is a due date on an assignment, no late assignments will be accepted. If a student does not turn it in on time, it is up to the parent to grade it. They will need to manually adjust it in their own records to give credit for it.