Hi,
I'm Rodrigo and I'll help you.
Changing this configuration will not increase your usable memory. Your usable memory is not the total available because part of it is reserved for the integrated graphics. Some devices allow to reduce it in the UEFI BIOS setup, but not all have this option.
To access the UEFI BIOS, turn the computer on and press repeatedly F2 or Del.
On the UEFI BIOS look for UMA Frame Buffer Size, if you can't find this option, it is not possible to change it.