Issues:
1) The current processor needs to be in a socket. If so, go to next step. If not, foget it.
2) You would have to totally disassemble the laptop. Not for the faint of heart…
3) Any upgrade would have to be supported by the BIOS. That you have to find out from the vendor.
4) Any newer CPU would have to be the same socket (of course), and the same thermal dissipation (or less). You can't put in a higher wattage CPU, as the cooling system can only handle the wattage it is designed for (even then, cooling is poor in laptops)
You can check this list (near bottom of linked page), for compatible processors. They seem to be all i5 types, just higher speeds. They are G3 sockets. You can't use any of the i7 listed, as their wattage is too high.
In my opinion, the best you can do is the i5-4340m, if the BIOS supports it.
http://www.cpu-world.com/CPUs/Core_i5/Intel-Core%20i5-4200M%20Mobile%20processor.html