the GPU is already overclocked, further OCing can damage your GPU. so be careful
anyway start increasing the GPU clock by 20-25 MHz and test stability, if it's okay you can go further until you become unstable, then you'll be needing to raise the voltage and that's what you don't wanna do....
check the temperature of the GPU after you overclock and if you experience a crash or a BSOD during games, go back to stock clocks
Edit: you'll overclock your GPU using MSI afterburner