That's something you normally do in your BIOS configuration. You need to talk to your board manufacturer regarding how to do this with their BIOS.
If you are running Windows, making this change will accomplish absolutely nothing. Specifically, it will NOT increase graphics performance. The Windows driver has built in support for dynamically growing and shrinking the amount of memory committed to graphics and, by default, will assign up to 50% of available memory to graphics if and when it is absolutely needed. By dedicating memory to graphics, all you are doing is ensuring that this memory is NOT available to Windows and thus, potentially, you could be lowering Windows performance.
If you want to do this because you have a game that will not run (that says there is not enough graphics memory), then this game has a BUG in it. Go tell the game producer to fix their BUG!