To reset a 6620L to factory defaults:
1. Confirm the device is powered on
2. Remove the back cover
3. Insert a paper clip into the Maser Reset Button hole
4. Press and hold the button for about 5-10 seconds
5. Wait for the Jetpack to reboot
More into on page 109 of the 6620L User Guide:
http://www.novatelwireless.com/files/6714/1236/6682/MiFi6620L_UserGuide.pdf