Are you using a graphics card or on-board graphics?
For on-board graphics you go in to your BIOS before Windows starts, go through your options, likely under Integrated Peripherals or similar, you'll see a Video RAM option set to 128M or similar, change it to 256M.
Normally on-board graphics isn't very good, so needing more than 128Mb is unlikely (any game which needs this probably will not play well).
If you're using a graphics card, you can't increase it, you need to buy a new card.