From the Main Menu open the Money section down the bottom. Select Items in the subsection.
This will bring up the List of all existing Items.
Click on the Item name or Pencil icon to edit the Item (note that the Pencil icon will appear when you hover over the Item row)
The Item Details Sidebar window will appear where you can edit all the relevant details for this Item.
Once done, click on the Save button and this will close the sidebar window and save the updated Item details.