The specific instructions for replacing your hot end will vary depending on the brand and model of your hot end and 3d printer. To change your hot end, you need the new part and any other tools instructed in your user manual. Make sure your hot end is completely cooled down before removing it. Take off any clips keeping it in place, the coupling collet, and the Teflon tubing. Next, unscrew the cover and take that off along with the hot end. Once the old hot end is removed, replace it with the new one and reconnect any part that you disconnected from the old part.