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.