If you want to start creating an assignment from scratch, you can set the deadline there. If you have an existing assignment where you would like to make changes to the deadline - you can find it from the Course Homepage either in the Course/Program Outline or in the To-dos section.
Upon clicking on the assignment you want to edit, it will open in a layover. You can then click on the gear at the top right to edit the assignment.
Hover over the Basics section and you should be able to make edits by clicking on the "Edit" button.
You can then make adjustments to the assignment release dates and deadline.
Don't forget to hit Save when you're done.
Note that if you change the release date of an already released assignment, any submitted student work will be lost.