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First, click directly on the name of the assignment, either on the Assignments page in Canvas or (if your instructor uses them) from Modules. You can also click on the name of the assignment from the Course Summary at the bottom of the Syllabus page.
When you are on the page for the assignment, simply click on the Submit Assignment button over on the upper-right, as shown below:
You will then see a Choose Files or Browse button. (The name of the button may vary, depending on your browser.) Click on the button and upload your assignment:
If you did this correctly, you should then see a Turned in confirmation message on the far right of the screen:
Consult our separate page https://community.canvaslms.com/docs/DOC-10666-421254359 for information on retrieving your instructor's comments.