You can only modify an assignment submission if your instructor has setup the course so that "passing" an assignment as determined by your instructor is a prerequisite to advancing to the next unit of the course.
If you are convinced that you should be able to modify your assignment, please reference this article:
As always, you are welcome and encouraged to get technical support or training by following this article: https://supportedemployment.freshdesk.com/support/solutions/articles/73000534763-technical-support-by-email-phone-or-zoom