i5 750 CPU Clock Ratio

[Aidan 1]

Im trying to Overclock my CPU, and what I believe you need to do is change your CPU Clock Ratio.

Mine says:

CPU Clock Ratio 2.66ghz ( 133x20)

I try to change the 133x20 to say, 133x21 but have no sucess, as it appears to be locked, my computer is custom built and not by HP or Dell etc, so I do not know what im doing wrong.