The lid" of the CPU, is a metal piece called the 'diffuser.' You can't take it off. It is metal and aluminum in colour.
But there could be an aluminum piece at the bottom of your CPU cooler that can come off the CPU's diffuser. The bottom of the cooler will be very well stuck, almost glued, to the CPU lid using thermal paste that can be removed, or possibly even a thermal bond that will permanently bond the two together.
If you have doubts, don't rip the CPU apart. Take it to a professional.