When stock arrives from the supplier, open the purchase order and check it.
You can use the Receive Tab to enter all items received. If you received a different item that wasn't in your order, you can still drag-drop it into the Receive list and mark it as received. You can also select a scanner to scan items in as they are received. This will increment the received count for the item and add it to the list if it wasn't on the original order.
The receive tab is ALWAYS used to receive stock, even for dates that already have stock on them, just add the additional items to the receieve tab, set the same date and hit "save"
You can receive more items and different items than you ordered, simply drag-drop the additional items into the list on the recieve tab.