In the main grade book window you have two default assignment sort options, "oldest to newest" and "newest to oldest." You also have the option of setting up a custom assignment sort order. You can choose your sort order in the "Filters & Options" window you get to by clicking on the filters & options button on your main grade book screen.
If you select "custom sort order" at this point you will not notice any change as you have not set up your custom sort yet. To set up your custom sort, go to the Grade Book ---> Assignments menu. On the "Assignments" screen you will notice a "Custom Assignment Sort" button. Click on this button.
A new window will appear that lists your assignments. In this window you can drag and drop your assignments into the order you want. When you are done sorting your assignments, click the "Save changes" button.