If you are using QuickBooks, you will need to add or modify the items in your QuickBooks company file first. Then, open the Core5 preferences and re-import the Items. This will not delete any existing items, but will update existing items and add any new items into Core5.
If you are not using QuickBooks, go to the Items tab in the Core5 Preferences. You will see a '+' button to the left of the labels. Press the '+' button and enter in the item name, item description, and unit price.