You can only do this if you have integrated graphics.
If you do, restart your computer and hit the key for the BIOS (F1, F2, Tab, or Del).
There is a section that says "graphics memory" with a number by it. In your case, 128 mb.
Your onboard graphics may only support using 128 mb.
If it is a discrete card, there is no way except to buy another card. If it is integrated then vram uses system ram and windows already shares an extra amount on top of the set amount. There really is no point in setting it higher since the set amount is still using the system ram.
Your gpu is most likely very weak which is the issue and more vram will do absolutely nothing.