If you want to change the cost category of an Expense because you forgot to assign it, or assigned it to the wrong cost category, you can do so as follows:
From the menu, select Expenses → Maintain Expenses and you will see the following page.
At the top of the page, you can look up the relevant Edition by specifying a Supplier, Year or Period.
In the example shown above, we see an Expense for which we forgot to assign the Cost Category.
We now double-click within the Cost Category field, after which you can select a Cost Category to assign to the relevant Expense.
After assigning the Cost Category, we need to commit the assignment by pressing 