How do I change how my expenses are categorised?

This screen decides which categories count as job costs (cost of goods sold) and which are overhead — tap each category on or off and the Profit page re-works its numbers; your expenses themselves do not change. It does not move a single expense. To fix one that landed in the wrong category, edit that expense on the Expenses page. DeskOps can remember the category for that vendor, and it will then suggest it on future bank and processor rows from the same vendor — you apply the suggestion with a tap; it is never booked for you.