If your instructor creates a cloud assignment with a Microsoft OneDrive file, you can open the assignment, edit the document directly, and submit your changes as your assignment submission.
When you open the assignment, Office 365 will create a copy of the file as your submission. You can open the document and make changes per the assignment instructions. When you are ready to submit the assignment, return to the Canvas assignment page and submit the assignment. The copied document is also added to your OneDrive folder for the course.
Cloud assignments are treated the same way as any other type of assignment submission. When you submit your assignment, your instructor will view the assignment exactly as it is when you click the Submit button. Even though cloud assignments can still be modified in the assignment page, any changes you make to the assignment after the submission cannot be viewed by your instructor unless you submit the assignment again as a resubmission.
Please review Submitting a Cloud Assignment for more details.