The assignment page will give you all the details of the assignment and it will also allow you to edit the assignment. Get there by clicking on any assignment name in the sidebar.
There are multiple tabs on the assignment page.
The Overview tab is where you can view basic details of the assignment including dates, description, and settings. If you need to edit these details click on the small gray pencil to edit the details. You can also select the Settings tab to edit, this will look much like the page where you first created the assignment.