Step 1: Requirements
- A Raspberry Pi
- A Windows PC with Notepad++ installed http://notepad-plus-plus.org/download/v6.1.8.html
- A small heatsink and thermal paste (If you're going to go past 750 or 800mhz)
The heatsink can be bought from Jaycar (if you're in Australia) or Radioshack (If you're in America) or where ever else you can find a Pin grid array (PGA) heatsink. http://www.jaycar.com.au/productView.asp?ID=HH8580
Step 2: Create a config.txt file
If you're feeling bold you can adjust them a little higher, but if you're going to go to 900 you definitely need a heatsink as well as bring the GPU down about 50mhz to make up for the power being supplied to the unit.
Once this is done, save your file to the boot partition of your SD card as "config.txt". The boot partition will be the only partition that shows up on Windows usually(about 60mb in size) - if it doesn't show up, you'll have to put your config.txt on a usb and transfer it onto the SD card with a Linux machine like Ubuntu.