To be clear, in general, changing generations of processors (Core/SandyBridge/IvyBridge/Haswell) involves a completely new socket interface (except for Sandy Bridge and Ivy Bridge). More details:
Core series (ie, i7-950 etc) = LGA1156 socket
Sandy Bridge (ie, i7-2600) = LGA1155 socket
Ivy Bridge (ie, i7-3770) = LGA1155 socket
Haswell (ie, i7-4770) = LGA1150 socket.
So - as you can see, if you have a Sandy Bridge system, you could upgrade to an Ivy Bridge (which was a decent improvement in efficiency and processing IPC), but going to Haswell would require a whole new motherboard. According to the parts list - you don't really specify what you -have- right now, so to just go right to the top of what you could do if you're looking at an i5 series, you could probably drop in an i5-3570. I wouldn't get a -K unlocked version as I'm willing to bet that Lenovo's setup doesn't really support overclocking so don't pay that unlocked premium.
Good news is that the i5-3570 runs about 20 watts LOWER power than the Sandy Bridge parts, so your computer will run a little cooler AND a little faster. Or a lot faster depending on which Sandy Bridge i5 you currently have.