1. Turn off the phone by pressing and holding the Power button.
2. Next, press and hold Volume Up and Volume Down and Power button for a moment.
3. A Boot Mode will start. When it does, let go of held keys.
4. Choose Fastboot Mode - navigate with Volume Up and select with Volume Down.
5. Your device is now in Fastboot mode. Superb. Navigate this mode using Volume Keys; select options with Power button.