Normally it's on the BIOS settings, but on most pre-built systems you can't change it.
There is still unconventional ways to unlock your RAM using SetPLL, but your motherboard's PLL needs to be in that list : http://www.overclock.net/t/601904/setfsb-clock-generator-list-compilation
Otherwise you might can't overvolt/overclock your RAM.
Normally it's on the BIOS settings, but on most pre-built systems you can't change it.
There is still unconventional ways to unlock your RAM using SetPLL, but your motherboard's PLL needs to be in that list : http://www.overclock.net/t/601904/setfsb-clock-generator-list-compilation
Otherwise you might can't overvolt/overclock your RAM.