Wedge the plastic opening pick in between the edge of the heat sink and the aluminum perimeter with care not to scratch the CPU beneath.

Gently pry up on the heat sink until it is loose enough for you to remove. Then grab and remove the heat sink.

If it does not work in the first position that you tried, try another. Do not pry too much in one position because it could damage the components beneath the heat sink.

Clean and Reapply the Thermal Paste during reinstallation.