Apple will not do it for you. Although it is supposedly possible to make the CPU change you must use one with the same pin arrangement as the socket on the motherboard. Then there would be the power increase and cooling demands. Disassembly is not easy and requires both knowledge and skill not to mention instructions (search for them on Google.) If you want the seventh gen. CPU then buy a new Mac. You may get enough money selling your old machine to pay the cost of buying the CPU.
Note that if you take the machine apart and make any unauthorized changes, Apple will not provide you with service should you need it. If you pursue your request be sure you know you are well-qualified to perform the work.