To change the product quantity for manufacturing orders while keeping the same quantity per unit ratio, click on the Change Quantities option in the menu. 
This opens a popup window where you can enter new quantity for the manufacturing order product. Set the New Quantity value and then click on the [CHANGE] button.
This option changes product quantity and recalculates and changes items quantities as well (based on the defined quantity per unit). For example, if you wish to change quantities and you enter new quantity that is half of the initial quantity, when recalculated, all items quantities will be reduced by half.
If you change the quantity directly through Quantity field it won't affect items quantities but it will recalculate quantity per unit based on the defined items quantity.