Before mac, I had a classic commputer with that big box and monitor. Now i have mac that is only a screen without a box. I bought new CPU and I want to add it, so how do I open mac
You mean you bought a new disk for your Mac? A Mac is an All In One meaning you don't need "the tower" alongside it.
But you can use a external disks and other peripherals as you would when you had a real PC.
You don't open it up and you don't install a new CPU. MAC processors are soldered onto the board. You DO NOT UPGRADE the components of your MAC. That is a non-MAC computer thing.
There is no open Mac. There is only closed Mac.
What you are referring to is an iMac, and as the other comments have already stated, you cannot replace the CPU on an iMac.
I would have hoped you had done your research BEFORE buying a CPU to make sure you had the correct socket size and generation information for that model of iMac. Such research would have told you that you cannot upgrade the CPU.
Which Mac do you have?
U BUY ANOTHER MAC
u open it