Here is how you can propose a modification to a Purchase Order, as a supplier.

Click within this box (1) to open the calendar in a side-panel. This is where you can select the new delivery date (2) you want to propose. 







If you want to propose a change in terms of quantity, click here (3) and type in the new quantity you want to propose. This line (4) is automatically ticked when an element, such as the quantity or date, is modified. You can also click on the red X button to to cancel the rejection before responding. When you're done, click on Respond (5).