I wouldn't use this solution.
It made many problems to me.
This solution delete the order only from `ps_orders` table but the order information stays in many other tables.
Example:
I deleted order number: 4 - with 3 products
Then I placed new order with 3 products and presta gave it id ...: 4
When I looked in orders tab the new order had 6 products instead of 3.
That was because information from old number 4 order wasn't deleted when I deleted order with described method.
So I don't recommend this solution.
I'm using this module after a few modifications.