There are many threads on overclocking, you should use the SEARCH feature....... or the thing called google.
But getting even MSI afterburner program will do the job i believe. You need to overclock slowly (moving up the values at 5-10 at a time) and then testing the video with FurMark or Kombustor again from msi to see if your card does any weird graphics effects to make sure you are not pushing it too far.
anyways... simple.. there are many threads explaining basics of overclocking your gpu.
I have the gpu OCed by +30Mhz, MEM by +500Mhz, Power offset 110%, and the voltage is up to 1000mV. This is the best I found while keeping the voltage in the green. I also adjusted the fan curve so @ 70c it is at 50% speed and goes linear up to 100% speed @ 80c. Never breaks 71c.