Thank you for contacting us. In most cases, you can change these values through BIOS level. The recommendation is to ask for help to your system manufacture.
This might depend on the type of system, if it is desktop or laptop. What happens in some cases with branded computers and laptops manufacturers do not include such setting in their BIOSes. I would like to share with you the following link, with some basics: Graphics Drivers — Graphics Memory FAQ for Windows 2000/XP/Vista/7*
Please post your system information and your BIOS version.