To individually update assignment due dates
1) Navigate to your assignment in Canvas and click Edit (you can do this after clicking on the assignment itself, or by clicking the three dots next to the assignment in the Assignments page):
2) Edit the "Due" field from there and be sure to click Save!
To bulk update assignment due dates
For more detail into this process, see Canvas's documentation here.
1) Navigate to the Assignments page by clicking Assignments in the sidebar
2) Click the three dots at the top-right and then click Edit Assignment Dates:
3) Edit due dates as needed from this page. To edit several assignments at once, check the boxes to the left of each assignment and click Batch Edit at the top. From here, you can either shift due/availability dates forward or remove them entirely.