How to OC on Asus H81M-C/CSM motherboard

Hi, everybody! I want to OC a Intel Pentium G3220 CPU which is currently running at 3.00 GHz. I can OC it to 3.9 GHz, but I cannot find out how to do that in the BIOS (it is UEFI) or AI Suite III. The only CPU options I can access is the voltage. Does the frequency go up with the voltage? Is there a hidden option that I could use? Thanks in advance!!