To upgrade from the V3 extruder to the V4 extruder, simply purchase your V4 Upgrade Kit and follow the instructions provided by the assembly guide, listed in the link below:
For M2 3D printers shipped since November 2015 (with metal extruder motor mount)— Download Assembly Guide - Dual V4 Extruder Upgrade.
For M2 3D printers shipped before November 2015 (with plastic extruder motor mount)— Download Assembly Guide - Dual V4 Extruder Upgrade.
The Dual V4 Extruder Kit Usage Instructions and Firmware are posted on the MakerGear wiki. Please note that you must first update the firmware on your M2 3D printer in order for it to recognize and allow control of the new extruder.