At 2.53 GHz I'm assuming that you have a Core 2 Duo T9400. If this is the case then yes, you can upgrade your processor. You just need to find another Socket P processor.
However, your processor is pretty much one of the fastest and most reasonably priced mobile processors out already. There are only 3 or so faster Intel socket P processors out right now. So it wouldn't be too great a speed boost or investment. Also remember that changing the processor yourself often will void your warranty, so check before. You can usually only change things such as hdd, ram, battery, and case cosmetics without voiding your warranty.