Override GST on a Purchase Order

Just trying to verify if I can override the GST on a Purchase Order. Currently all options are greyed out.

I can see the GST option on a Purchase Order > Purchase TAB > Tax >GST button. But when I select the button which shows current GST amounts there is no way of editing the amount. The Purchase Order is a current Open Order.

Worked this out as had been trying to edit PO line before the PO Invoice is generated. GST can be adjusted at the invoice level before it is posted.