To change codes:
Admin/Expense/Expense Admin/Accounting Administration
This is where all "codes" are mapped and you can find the item that you need to change in this area.
Merchant Code mapping is done by going to Administration>Company>Tools then choose the Company Card option. This will take you to a screen with several tabs, one of which is labeled Merchant Code Mapping. This is where you can map a certain merchant code to a certain expense type.