After you create one (or more alarms):
Tap the Alarm icon (to the right of the alarm) to between enable between enable (blue) and disable (gray).
Tap the alarm time to change any of that alarm's settings and save it.
The Alarm/Clock app has four tabs across the top of the screen giving you access to a variety of time functions. You can tap Alarms, Timer, World clock, or Stopwatch, or swipe horizontally to scroll through and access these functions.