Use strong, steady pressure to release the heatsink from the motherboard in the next two steps.

Insert the flat end of a spudger between the top edge of the heatsink and frame.

Pry the heatsink up at different points along the top edge to release it from the motherboard.

If the heatsink won't budge at all, use a hair dryer to warm the heatsink and soften the thermal pads.