If you want to start creating an assignment from scratch, you can set the type of deadline there. To make changes to the deadline of an already posted assignment, you can find the assignment from the Course Outline or specifically in the To-dos section on the Course Homepage.
In this example, we will edit the deadline of the assignment titled "Assignment: Leadership Reflection".
Upon clicking on the assignment you want to edit, it will open in a layover. By clicking the gear in the top right, you will be able to edit the assignment.
In the Basics row, hovering over this section will allow you to click on the Edit button, as shown below.
After the noted deadline, if the box is checked, the deadline is a hard deadline. Unchecking allows you to make the deadline soft.
Don't forget to hit Save when you're done.