It depends on your laptop more than anything else as to whether or not it is physically possible, while the CPU is removable is is entirely possible that the heatsink holding it down may not be. In most cases it will be upgradable but laptops are made on a case by case basis.
Due to that is it advisable that you DO NOT upgrade a laptop CPU. The cooling system is designed to be able to cool that specific CPU and the heat that that CPU typically outputs. Usually an upgrade means more power at an increased TDP, which means it runs hotter and laptops have enough cooling issues at stock than to worry about adding more heat.
However if you really feel like upgrading the CPU, upgrades can theoretically consist of many CPU's that share the PGA998 socket such as the i3 330M, the i7 920XM and the i5 520M. However before upgrading I suggest double checking that the CPU you choose will work on your specific laptop, it's not uncommon that the some CPU's just plain don't work with certain laptops even though the CPU should be compatible based on socket
-Member of the Intel Response Squad
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