Navigate to Network⇒Inventory⇒Part

Double click the part that you want to amend the stock levels of

Select the Stock-In or Stock-Out option as required

Select Stock In (or Stock out if reducing the stock) and, in the pop-up window, enter the quantity the stock is to be increased/decreased by and location (and any further information as required) then click Save

The stock level has now been increased/decreased by the specified quantity