Answer: 

The Last Cost field on the product record is an auto-calculated field and will be the cost on the latest purchase order or build of that product. 

To update the Last Cost please see the below:

1. Navigate to Inventory>Products>Import/Export

2. Products tab

3. Click the drop-down menu next to Export in the top right-hand corner and click Products. This will download an excel file (make sure not to change or delete any information as this will be reflected in Unleashed)
4. Scroll across to the column Last Cost (Column AJ) and enter your new Last Cost
5. Save this document as a CSV file
6. Return toImport/Exportpage in Unleashed.
7. Drop a file here or click to upload your newly saved file

You can always double-check a file has been imported via the 3 parallel lines next to your name in the top right-hand corner of the screen.