Begin by powering off your device using the Power key and selecting power off.
When your device shuts off, hold the Power key + Volume up together till you see DEXP logo.
By now you should be in Recovery mode and from here you will use Volume keys to navigate, so select "Wipe data/factory reset" and confirm it with Power button.
Confirm your decision once more by selecting Yes and wait for the process to begin removing all personal data.
