To get the default settings on your device:
Go to Settings -> General -> Advanced Settings -> Enter your PIN -> Reset settings.
Alternately, you can long press the power button on your device till the screen goes black or press the button on the side 5 times in quick succession.
This will initiate the setup process.
Please note, you will lose all your custom settings, reminders and credentials of On Demand apps after this process. You can enter them back when the setup is completed