Hello @Mary Frances.M.273
Thanks for connecting with the Brightspace Community.
To Submit/Upload an assignment you can follow the below steps
--Click on Activities & Assessments.
--Find the desired assignment and click on it.
--You will see your instructor's description of the assignment here.
--Scroll down to Submit Files. ...
--Wait until your file has uploaded completely and you see the File Upload Results page.
Here are tutorials that explain how to submit an assignment and post a discussion