Impossible. The card is soldered onto the motherboard itself. Upgrading would need you to get a completely different mothermoard. And even so, it is not just the core, the memory, and the other components that it depends on would also need to matter. It is not just about the core.
Asus G46VW
i7 3630QM | 750GB Hard Drive | 1080p Full HD Display | Corsair Vengeance 16GB | GTX 660m | Windows 7 Ultimate