Your local vape shop has a hardware box for a programmer, which interfaces between a PC and the P3. To update the Radius, you had to install the USB driver. There is no driver for a direct programmer, thus the need for the programmer box. Look up JTAG if you're curious. I haven't taken a P3 board apart, but would venture a guess that's what they're using. Maybe Vape360 would be willing to share some technical secrets?
PS - If it were possible to use a direct USB cable, they probably would have, but since the vape shops have a $200 programmer box, logic dictates it's not possible to do at home without the same box.