Use this adapter plate in the combination with http://www.thingiverse.com/thing:559657by Jimmy Hartanto for the ultimate security and stability of your new and shiny E3D V6 hotend in your Printrbot Simple Metal.

Installation can be fiddly - one of the screws is hard to reach and I recommend keeping the fan off during the installation. It uses the same 3 screws that hold the Extruder to the frame. It is recommended to use 1-2mm longer screws but the plate is designed to work with factory screws without any problem. I don't think I have to explain anything more - you should be able to figure it out. :P (the protruding bit goes to the top and into the hole in the frame with the hotend)