I think what you are looking at is the watering duration for the instant manual run (not the user program) in the zone sections shown on the home page. The system remembers the last duration of the instant manual run and displays that on the home page. The watering time for the program remains unchanged. You can go to Settings->Programs or Settings->Schedules\ to verify.
Please tap on the "15:00" text and there will be a window pop up for changing that value.
You do not need to change the value for the instant manual run. The watering time for the program overrides and remains unchanged. On the home page, the system always remember the values of the last manual run but not the programs. You can go to the Schedule page to check your programs.