Why am I not able to change the approval Status of the order from Pressero Admin?

When the order is placed, and no approval plan is active the default approval status of the order item will always be approved. Once the approval plan is active then the approval status of the order will depend upon whether approver has approved, denied, or approved the order with modification. You can change the status of the orders that falls under approval plan, however any order that was placed before the approval plan was active will always keep Approved status and it is not possible to change the approval status of these orders.