Apparently you can't lower the CPU voltage on that mobo according to this manualhttp://www.iusermanual.com/motherboard/asus-m2n68-am-plus-motherboard-manual.htm
Topically to enter the BIOS setup you press [DEL] right after you power the PC and in there you have a menu - you navigate in there using the arrow keys and hit enter to set an option or value. You can also use the numpad on the right of your keyboard to set some values.
For the CPU voltage you go to Advanced then CPU configuration the Processor voltage and change from Auto to a lower voltage but all you have in there is a "Power saving mode". F10 is for saving the modifications and exit to windows.
You can also remove the CPU/heatsink, remove the paste (sanitary alcohol is fine) and use a little good thermal paste like the Arctic Silver 5. http://www.youtube.com/watch?v=fRRWbQUqW1Y