It cannot be increased -- the system has no dedicated video RAM.  The integrated GPU uses your system memory.

If you're running an application that requires dedicated video RAM, you need a system with an nVidia or AMD/ATI GPU aboard.  You would have to replace the system board with one like this:

https://www.parts-people.com/index.php?action=item&id=15703

(and  you'll need a new heatsink assembly and likely a higher-power AC adapter to go with it -- the one that comes with the Intel GPU board doesn't fit the mainboard with the discrete GPU;  additionally, while a 65W adapter will power the integrated board, you need 90 or more to power the discrete mainboard).