Hi brummyfan,
I don't think that you can change this (unless the option is there in your BIOS - check the manual).
Dedicated graphics memory refers to memory that is included on a graphics card, whereas Dedicated system memory refers to memory that is assigned to an onboard graphics solution.
The Total available graphics memory is made up of Shared system memory and either of the above, depending upon whether you are using built-in graphics or a graphics card.
When you increase your main system memory, it is possible that the amount of Shared system memory will increase, but that depends on the system architecture and options available in your BIOS.