What is the make and model number of the fan as you have selected a desktop fan as the device and not a "tower fan"?
If the fan is no longer under warranty, you may have to open the fan and access the control board where the speed buttons are connected to and check the components. Usually with speed controls for fans, the switches connect or disconnect capacitors into the fan's motor circuit to increase / decrease the amount of current flowing to the motor so that it runs faster or slower.
If you see any problems or if you're not quite sure, post some images back here.
@sachin gupta
What is the make and model number of the fan as you have selected a desktop fan as the device and not a "tower fan"?
If the fan is no longer under warranty, you may have to open the fan and access the control board where the speed buttons are connected to and check the components. Usually with speed controls for fans, the switches connect or disconnect capacitors into the fan's motor circuit to increase / decrease the amount of current flowing to the motor so that it runs faster or slower.
If you see any problems or if you're not quite sure, post some images back here.
Here's how to do this [guide|21499]