Its not dedicated video ram, rather some of your system ram being reserved for your intel HD graphics.
Some BIOS settings may allow to you reserve more ram for your Intel HD, but if you dont have an option for it in your bios, there is no way to change it.
In reality, allocating more ram to your IGPU isnt going to make Intel HD perform better.
The Core i5-2400 can use up to 32GB of dedicated RAM.
The Graphics Video Max Memory as the maximum amount of system memory available to the Integrated Graphics Device (IGD) as Video Memory (VRAM). However, "IGD Aperture Size" and "Graphics Video Max Memory" aren't interchangeable terms.
IGD Aperture Size is a definition for the size of the Graphics Translation Table.
A large IGD Aperture Size is not a good idea 100% of the time, since this is space that would be permanently reserved. So it won't be available to the OS other than for Graphics processing purposes.
There's no default ‘one value fits all’ for IGD Aperture Size.
Increasing what an i5-2400 already uses won't make much diference. I suggest getting a cheap video card that has its own memory on the card. You can move up to a better video card from there.