The on-board Inventory Management system can be used as simply or as robustly as you would like. You can sell products using the system without ever bothering to manage quantity, etc., or you can have the system keep detailed records re quantity-on-hand, etc. If the later is your case and you want to add more product or otherwise adjust the quantity available (say you just got a new shipment in), here is how you do it:
Click on your List Inventory option on the Home Dashboard:
Select the inventory item you wish to modify:
Once the inventory item screen has opened, click on the Receive Inventory tab:
Make your entry and Save For Entry Type, select whether this you are Receiving new goods or making an Adjustment to correct for the quantity-on-hand. The only other required field is the Quantity. Save your changes and your inventory quantity will be adjusted.