If you're asking for step-by-step instruction, you're going to have to provide your motherboard make and model. Otherwise, get into the BIOS; find the section where you can adjust your DRAM settings (this is often under a tab labeled "Advanced"); make the adjustment; save & exit.

Note that increasing the RAM frequency also increases the CPU frequency, as they share the FSB. I also noticed that you're using two different RAM kits. You may encounter BSODs due to the mixed RAM. If you experience BSODs, try booting with just one kit. Finally, if you're not using a 64-bit OS, you won't be able to utilize the 8 GB of RAM installed, even if you don't get BSODs.