well first of all is do some research. see if anyone on youtube has tried to overclock a 620. google gtx620 overclock and see if there are tutorials on your specific card. when you feel confident enough try afterburner and use small steps in a gpu intensive game. i use 2mhz jumps. when you get too high your game will have artefacts and bad graphics...dial it back down a bit till its stable and it should be good.
Okay thank you, I was tweaking around with the settings and figured it out. I clocked the core from 710 to 900 and I got 10 more frames in Skyrim. Although the temp went up 5 degrees it's totally worth it.