Follow the steps in this guide to remove or replace the heat sink in a 21.5" Retina 4K 2017 iMac.
Some images in this guide use a 2015 iMac, which has minor visual differences. These differences do not affect the repair procedure.
Make sure to follow our [guide|744|thermal paste application guide] after you have removed your heat sink.
Opening this device requires cutting through adhesive that will need to be [https://www.ifixit.com/Guide/iMac+Intel+21.5-Inch+Retina+4K+Display+%282017%29+Adhesive+Strips+Replacement/96196|new_window=true|reapplied] after you finish the repair. Be sure to test your new parts before sealing the iMac with the adhesive strips.
This guide is marked "potentially dangerous" because it requires you to handle a power supply that contains large capacitors. Unplug the iMac and hold the power button down for at least 10 seconds to help discharge the capacitors. Handle the board by the edges and do not touch surface components.