Open Finance → Overview, scroll to the month table and click the pencil icon next to the desired month. Enter the correct amount for income and/or costs and save.
The override fully replaces the automatically calculated amount for that month. In the monthly graph this month gets a distinctive marker (striped bar or icon) so you can see at a glance which months have been adjusted.
Useful when you've received an ad-hoc retainer, want to correct a missed invoice, or are importing historical data.