Where can I add or edit the expense categories?

When you add an expense, you are able to select a category for the expense to help with filtering on reports or your bank reconciliation. Default categories already exist, but you can new categories or edit the existing to customise for your company. 

To do this, you would click into Admin, then in the row below System Data:  

Under Common Data Sets, you will click View next to Expense Categories:

You will then be able to add new categories as well as edit the existing ones.