Locate the service manuals or some kind of tear down. Confirm that the process will be within your capability. Order some thermal compound. Disassemble the laptop until you remove the heatsink from the cpu. Clean the old cpu and heatsink with isopropyl until it’s as clean as can possibly be. Apply new thermal compound. Reassemble laptop.
A1502 could be a lot of laptops, use the emc number or serial to find out which one or just look for the MacBook Pro NN,n number in the about option under the Apple menu. It doesn’t matter which one you have, they’re all really easy to work on and well documented.