Thermal paste bonds the bottom heat sink fins (closest to the power button) to the metal shield. Depending on the age of your device and condition of the thermal paste, separating the heat sink may take some force.

Insert the flat end of a spudger under the bend in the copper pipes, near the heat sink's bottom edge.

Use your spudger to pry up the heat sink, applying steady pressure to separate it from the thermal paste.

If the heat sink doesn't fully separate, use your spudger to pry up the bottom edge on the other side of the metal fins.