(unless your specific device documentation requires a volume key boot-trigger).

He connected the phone. A red bar shot across the bottom of the screen: DA Download 100%

Turn off your Transsion smartphone completely. Hold the Volume Down (or Volume Up) button and connect the phone to the PC via USB. The tool should detect the device and assign a COM port. Step 6: Begin Flashing

The safest way is through the official Carlcare App or Transsion support portals.