The Adjust Inventory function allows you to change inventory quantities either up or down. This is the most common way to adjust inventory in the system for a given item and day rather than enter in new inventory.
Pre-Requisites
Access the Inventory module.
Click Inventory Adjustment.
Specify the Inventory Type.
Optional: Enter in the Item Code or click on the ellipsis to return a list of items for that Inventory Type. If you leave blank it return available inventory for all items in type.
Select an Inventory Location.
Specify an As of Date. This will return inventory as of that date and the adjustment you enter will occur for this date.
Click Retrieve.
Click the Item that you wish to adjust.
Click Adjust Inventory.
Locate the specific line of Inventory for this Item that you wish to adjust. Take a look at the available qty/pack as this is the most that can be adjusted as the 'As of' inventory may be allocated elsewhere.
Click in the Adjustment Qty or Adjustment Pack column.
Specify either a Quantity or Pack value. It should auto-calculate the other.
A positive number will add inventory, a negative number will subtract inventory.
Select an Adjustment Code on the line.
Specify an Adjustment Date.
Select a Post to Period option.
Click Save.
Click Close.
Repeat step #8 to #17 for the next item you wish to adjust. 
Click Save and Close.
You have now adjusted an inventory item as of a date.