If there is no fan on gpu don't do anything! Just get a better gpu if you need more performance!
GT 610 is slower than many integrated graphics on intel and amd processors. If you seriously want to overclock it then do the following:
- increase core clock at 5mhz steps until something happens (crash, artifacts etc.)
- when problems appear back down the clock 5-10mhz
- increase memory clock same way but in bigger steps of 10-20mhz
- when problems appear back down 10-20mhz
Play a game or run stress test after each adjustment and look at the temperatures.