Lift the heat sink away from the motherboard to remove it.

If the heat sink feels stuck, check for heat pipes caught underneath case overhangs, screws you forgot to remove, or stickers or tape.

The existing thermal paste may still be tacky. It might help to gently wiggle the heat sink side-to-side to separate it from the thermal paste holding it down.

Don't pull on the heatsink too hard; the heat pipes may be susceptible to bending or creasing.