with laptops you are limited to what you can update. I would not personally bother changing the cpu unless it had died. If it is seriously underpowered then sell it and buy a better one. I did not see if you are running 32-bit or 64-bit. If 64-bit can you upgrade the ram? go to crucial.com/uk and run their memory scanner it will tell you what is fitted and what you can replace it with. If it is 32-bit you already have more than it can use.
Surface Pro 5 News - release date, UK price, features, specs