There are multiple ways:
1. Press and hold the reset button for 5+ seconds. Look at the signal strength LEDs - when they're all lit up, release the button.
2. Enter the WebUI and go the the System → Administration → General page; scroll down to the bottom of the page and click the 'Restore button'
3. Use SSH or CLI to login to the device and executer these commands:
All possible methods with instructions are listed here.