Flip the main board over.

Lift (or pry with fingers) the heat sink straight off of the mainboard and processors.

Force will be required as the thermal paste "glues" the heat sink to the processor.

Note: If new thermal paste is readily available, clean the old paste off well and apply new thermal paste at the time of reassembly. If new thermal paste is not available or you are cheap, do not disturb the "old" thermal paste.