You can't. 97-98% of all laptops have soldered/integrated chips. They can't be upgraded or changed. It is what it is. Sorreh :| You can find a better lappy, though. HP makes nice laptops. Try to find a 8200m, 9600m, or GT120m. Those are all very nice mobile GPU's.
GT120m is a little slower than the 9600m. You can also get a 9800GTm, GTX 260m and 280m. Those are rare since you can't just "upgrade" to it. I was browsing some alienware lappies today with GTX260m and GTX280m options. Hell, you could even SLI them. The price tag reflected it though... :|