Cost/Unit, Supplier, and Date Purchased can be updated individually on the main Inventory page if you do not have any Replenishments for that item.
First, click on List at the top of your screen and select either FBA or Merchant, depending on what channel your item is in
This will take you to the Accounting view of the Main Inventory page
You can search for the item you'd like to update by using keywords, ASIN, or MSKU in search box showing in the upper right hand corner
Click directly on the area you would like to update and the Cost/Unit, Supplier, and Date Purchased fields will all become editable
Type your updated information directly into the boxes and click Save
If you have a lot of items to update, you can also import your buy cost in bulk using a .csv by following these instructions to save time!