There are a couple of ways to do what you are asking:
- A quick way to blow away all overrides for a set is through the Instructor Tools page:
- Click on "Instructor Tools"
- Highlight all students, and select the set
- Click "Edit one set for users" at the bottom
- Save changes
This will undo all of your overrides (though I have only lightly tested this), as it is effectively assigning blank User Values to all users. Now all students will receive the class value.
I recommend the first suggestion. The second one means that all students will have an override date, and the global dates are left as-is. This means that in the Homework Sets Editor you will see dates that don't apply to any students, which could easily lead to confusion.