Flights are automatically merged into trips if the arrival airport is the same as the departure airport, and the layover time is less than 12 hours. If the layover is more than 12 hours, you can merge them manually.
You may want to merge two flights that are parts of the same trip (e.g. Singapore > Hong-Kong > Tokyo).
Tap “Edit” at the top right-hand corner on the main screen.
Then select the flights and tap the "Merge" icon arrows on the bottom bar.
Tap and hold on the flight you want to merge, then select the other one. Tap on the "Merge" button at the popup.
You can easily split flights.
Swipe left on the flights you want to split.
Then tap on the "Split flight into flights" button to split.
Tap and hold the flight you want to split, and then select the "Split" button at the popup.