If you have deleted items from your inventory and you need to remove the item numbers from those items so they can be reused, go to the Inventory module (1) (which automatically brings you to the Items sub-module of the Inventory module), click the "..." drop-down menu button (2), and select Cleanup Old Items (3).
When you select this option, a dialog will pop-up making sure that you want to proceed with cleaning ALL of the deleted items. Click the OK button (4) to proceed.
To verify that this has been completed, a notification at the top right of the screen will let you know if the cleanup was successful.
In order to verify that the item numbers have been removed from the deleted items, click the "..." drop-down menu button (A) and select Manage Deleted Items (B).
As you can see below, the UPC/EAN/ISBN column is now blank. Now you can reuse the item numbers that were associated with these items.