Yes, it is possible for you to upgrade the processor. In fact, most of the components of your laptop can be replaced. The largest unscrew-able part under your laptop holds the processor, alongside the RAM and HDD. To get to the CPU, you may also have to remove the heatsink, or other obstructions.
Personally I think you're making a great choice by upgrading the processor, the P6100 is weak in performance, and it's even slower than my father's old Core 2 Duo laptop. According to the overview of your laptop on Lenovo, these processors usually come shipped with the laptop, so these are guaranteed to work:
Intel Core i5-520M
Intel Core i5-430M
Intel Core i3-350M
Intel Core i3-330M
Those above CPUs are usually shipped with your laptop, so again those are guaranteed to work. To be honest, your chipset does support i7's too, so you don't have to upgrade to one of those processors above, as long as the CPU you purchase is on Socket G1 (often called rPGA988AG1, PGA988, or PGA for short.) Be careful that the processor you want could also be for the BGA socket; which will not work with your motherboard, so ensure it's PGA.
If you update your BIOS (last available update was in February 2011), I believe you'll be able to install some higher end i3 and i5 processors, as well as i7's if you're willing to upgrade that far. Personally the i3-330M is definitely an upgrade for your laptop, but it may only be minor.
You could upgrade it to a better i3, or better yet, an i5. The best i5 which I believe will be compatible is the Intel Core i5-580M, and the best compatible i3 would be the i3-390M. i7 wise, you maybe able to safely install the i7-620M and i7-640M; you could also try other i7's on your laptop which I believe may work too, but can't guarantee it's compatibility; personally I do think your laptop can take better i7's.
All the best, and please let us know if you have any more questions, or how your upgrade goes. 😉
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