In standby mode, the application screen shows the current time.

To enter setup mode, touch the screen.
The top button allows you to select 12 or 24 hour time format.
Use the top slider to set the clock size.
The lower slider sets the noise threshold, when exceeded, an alarm sounds. The further to the right the slider is, the lower the threshold.
Use one of the three color bottom buttons to select the alarm sound.
To exit the setup mode, tap the screen.

Keep in mind that your phone or tablet will not go into sleep mode while the app is running, so it's best to keep your device connected to a charger.