By default, you can't change the unit type from the standard user interface, as changing from, for example, base to case or display, or vice versa, affects the product hierarchy.
As a superuser, you can make changes using a simple Excel spreadsheet.
See the attached Excel example.
In this example, the unit type for GTIN 5777188031317 is changed to "CASE".
Similarly, you can change in Excel to:
Base unit: Enter "BASE_UNIT_OR_EACH" in Excel
Display: Enter "DISPLAY_SHIPPER" in Excel
Pallet: Enter "PALLET" in Excel
Note! Be aware of fields D8249 (GTIN for the next lower unit) and D8270 (Quantity of the next lower unit) if changing from "CASE" to "BASE_UNIT_OR_EACH" - these fields must be removed from the product document before the Excel sheet is uploaded.