Insert the flat end of a spudger between the CPU and heat sink.
Gently pry the CPU up by slightly twisting the spudger.
Remove the CPU from the heat sink. Be careful not to touch the contacts.
Install your CPU into its socket on the board, not on the heat sink. Check the orientation and make sure it lines up correctly in the socket. Follow the instructions specific to your CPU type to apply fresh thermal paste to the surface of your CPU. Then, reinstall the heat sink over the top.