Once you have invoiced the customer order, you cannot reverse the customer order to Partially Delivered. However, if you add a charge line, the customer order status will be changed to “Delivered”. 

If you are using an IFS version APP9 or later, you can use Sales\Shipping\Undo Customer Order Delivery functionality to bring back a ‘Invoiced/Closed’ Customer Order back to ‘Partially Delivered’ state.

Only thing is that your Invoices should be in ‘Preliminary’ state and you should cancel them first, and there should be more than one Delivery (I.e. Order should be delivered not as a one full delivery, but as Partial deliveries - I hope this is same in your case as you want to bring your Order back to Partial Delivered state, not to Picked state). See below example I created in one of our IFS Internal Reference (Apps9) environments (Same can be done in later versions like APPS10 also),