In my experience of using laptops, some of the CPU and GPU are wielded on the motherboard and can't be replace. That's why you have to choose the suitable components when you are purchasing a laptop.
Regarding your questions, it is doable and I think it depends on what laptop you are using. You can go to the manufacturer website to check your CPU socket and whether it allows you to upgrade your CPU.
Ok let say your laptop is compatible, it doesn't mean that you have the green light to upgrade it. Tearing down your laptop to get access to your motherboard is not easy and as I mentioned before, it all depends which laptop you are using. Please do some research about your laptop to find any successful case and do it with some who KNOW WHAT THEY ARE DOING!