1. First things first: you're going to need a copy of Marlin.
If you are upgrading an existing 3D printer to use a V6 HotEnd, you should try to get a copy of your current firmware from your printer's manufacturer.
If you're building a new printer, or simply want to upgrade to the latest version of Marlin, download it at http://marlinfw.org/meta/download/
If you download a fresh version of Marlin you'll have to configure more settings than the ones mentioned in this guide so that it will work well with your printer.
2. Almost all printers use Arduino IDE to upload fresh firmware, so download it at:
https://www.arduino.cc/en/Main/Software
3. Unzip Marlin from the zip file you downloaded and put the resulting folder anywhere on your computer for safekeeping.
4. Inside this folder, navigate to the Marlin sub-folder and open the Marlin.ino file. This should open every file in Marlin.
Find the tab:
Configuration.h
5. In the tab:
configuration.h
find the Thermal Settings section. Below the comments, you'll find the settings for the types of thermistors your printer uses. (Typically there will be one per hotend and one more if you have a heated bed.)
If you're installing your Lite6 as your only HotEnd, change the first highlighted line to:
#define TEMP_SENSOR_0 5
If you're replacing an existing HotEnd or have multiple HotEnds, adjust whichever line corresponds to the tool number that you're changing (they start counting from 0)
6. While the metal of the V6 HotEnd can withstand very high temperatures, the PTFE lining of your Lite6 can only go up to ~245°C for extended periods.
This includes the use of 'high-temperature' tubing such as Capricorn' never exceed 245°C with the Lite6.
7. Set that as your maximum temperature by changing the first highlighted line to:
#define HEATER_0_MAXTEMP 245
(or adjust the line corresponding to the hotend you're changing.)
8. For safety it is strongly recommended to make sure that your printer detects if the thermistor stops sending correct temperatures for any reason.
Set the first highlighted line to:
#define HEATER_0_MINTEMP 5
(or adjust the line corresponding to the hotend you're changing.)
In newer versions of Marlin there are other features such as Thermal Runaway Protection that might be useful as well, though they are typically on by default.
9. Upload the new firmware to your electronics as you normally would. Typically this means plugging in your printer to your computer, selecting the correct COM port and board type, and pressing the upload button.
10. If you're unsure of how to update your printer's firmware, check with its manufacturer.