can we modify fan speed out of stock speed
 

is there another way to get high speed fan without buying new one

You mean turn a standard fan into a high-speed fan? You can overvolt the fan, but you risk burning it up or wearing it out faster...

how to overvolt the fan?

DO yourself a favour, and buy a new high-speed (or better: high flow/high pressure) fan instead of overvolting an old one.

i already buy fan from cooler master at 4300RPM but how can i overvolt the other fan

Why in god's name do you need more than 4000rpm? Hell, 40mm fans run at 4000-5000rpm, 80-120mms run at 1000-2000 for a reason. I dont know the size of the fan your trying to overvolt but most likley you'll just end up creating more noise and not even a -1c gain.

i just love speeeed man

Most all desktop fans run on 12 volt which is highest DC voltage rail the power supply provides; you cannot overvolt; you can undervolt.

The same applies to your $10 bill; you cannot buy anything over $10, you certainly can spend less.

Even if you find a way to overvolt; it won't be long you will have to replace the dead fan.

actually i want high air flow-low speed fan

doesn't everybody?