Probook RAM is 6GB - 2.63GB usable - How to increase it?

hi,

i have "HP Probook 4540s"

CPU - Intel Core i5-3230M 2.6/3.2 GHz

Memory - 6GB 1333 MHz DDR3 SDRAM

Graphics - AMD Radeon HD 7650M (2GB dedicated DDR3)

only 2.63GB of RAM is usable, how to increase it? is that possible?

I can't find in BIOS Memory Remap.

thank you very much,

Nikolay