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