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 time is more than 12 hours, you can combine them manually.
You can merge two flights that are part of the same trip (for example, Singapore > Hong Kong > Tokyo).
Tap "Edit" in the upper-right corner of the Home screen.
Then select the flights and tap on the merge icon arrows in the bottom panel.
Tap and hold the flight you want to merge, then select another one. Tap the "Merge" button in the pop-up window.
You can easily split the flights.
Swipe to the left on the flights you want to split.
Then tap the "Split flight into flights" button to split the flight.
Tap and hold the flight you want to split, and then select the "Split" button in the pop-up window.