acer aspire 4520

Solution
Basically, you have a Turion CPU. You can install a faster Turion, but you cannot install a different CPU with a different microarchitecture.

The cost of buying a faster Turion will not be worth your money, and installing it will not be worth your time. :)
There are other CPUs that are compatible with the socket your laptop uses, but it looks like the Turion X2 you have is one of the best processors the laptop will accommodate. You might be able to install a faster Turion X2, but the cost of upgrading the CPU from a Turion to a slightly faster Turion probably won't be worth it, and that is IF Acer's BIOS on the 4520 will support such an upgrade.