There's nothing you need to do. It adjusts automatically.
And you really want the amount of graphics memory to be as little as possible -- the more memory the graphics subsystem uses, the slower it goes because memory accesses are the major bottleneck in shared memory graphics systems. Using more memory improves image quality, but performance takes a massive nosedive. It's a tradeoff, it's not a "more is better" thing.
Update: PES versions 2009-2012 had a bug that caused them to generate an error with shared memory graphics solutions. A patch is available for 2012.
"As I said above, Pro Evolution Soccer 2011 (PES) has a bug where it incorrectly looks at dedicated vram versus total available graphics memory. If the platform has BIOS option to bump up the dedicated vram, you can workaround this application bug. If not, then you are stuck - you should contact Konami and ask them to patch the game. If they are unwilling, then I recommend you let them know you will be choosing to play games from their competition in the future (FIFA 2011, Football Manager 2011)." - Here