Please cut the power to the ChromaHead before starting the assembling and disassembling.
You may have several reasons for replacing the heater and thermistor. One reason could be if the cartridge and thermistor are not functioning. If the fan is not working, you can perform the necessary tests and replace the fan as well. You can also follow the same procedure if you want to install a different heater and thermistor.
The replacement of the hotend heater and thermistor is shown in the video below.