Making changes to your assignment is very simple. You can do this by going to your Timeline then clicking on the assignment. This will take you to the grading window. From there, click the edit button on the top left pictured here:
Once you click the button, you will get the option to edit the assignment it.
On mobile, simply tap "Class" then tap on the desired assignment. On the top right, there is an option to edit the assignment.
Note: You can only remove students from assignments if you have not graded them for that assignment yet.