Use a spudger or your fingers to lift the heatsink up and off the motherboard to remove it.

You may feel a bit of resistance. This is normal, since the heat sink is slightly bonded to the CPU with thermal paste.

Clean off the old thermal paste from the heat sink and CPU using high-concentration (90% or higher) isopropyl alcohol and a microfiber cloth. Apply new thermal paste to the CPU before reassembly.

Apply thermal paste to all surfaces that had thermal paste applied previously. This includes between the heatpipe and aluminum shield, which the Switch uses as additional heatsinking.