You cannot increase the dedicated memory. The Graphics driver allocates and deallocates system memory, to use as video memory (VRAM), based upon the demands of the currently executing software. Up to 50% (half) of available system memory can be allocated at any point in time.
If you have antiquated software (game) that demands that the initial dedicated memory be some specific fixed value, this is a bug in this software (game). You should report it to the software (game) developers for repair.
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