Are you sure you are at the right properties. If you are really on eclipse you should be able to click on Window/Preferences and see this window:
Expand the options like you see it at the screenshot. Here you actully should be able to edit your tasks!
If you want to change the priority, click on the left side of the editor and click on add task. With this task you are able to click on complete and change the priority. For more information look here.