Cost/Unit, Supplier, and Date Purchased can be updated individually on the replenishment detail pages.
First, click on Inventory at the top of your screen and select either FBA or Merchant, depending on what fulfillment 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, Title, ASIN, MSKU, or scanning the UPC in search box showing in the upper right hand corner.
Click directly on either the MSKU, Active Cost/Unit, Supplier, or Date Purchased.
It will take you to your sort sequences already in editing mode. The Active sequence will be labeled "Active" under the Status column.
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!