How RPM of fan can be increased?

That depends on the type of fan. If it's a belt driven fan, you should be able to change the drive belt pulleys.

If it's a direct drive fan, you'll need to change the motor.

Note an exception: some direct drive fan motors have multiple connection points which provide different operation speeds. When the cover plate is removed, there is typically a chart showing how to wire the motor for different speeds. If you see no such chart, you probably cannot change the speed of the motor.

Also note: most modern fan motors are synchronous type, meaning that they control their speed from the fact that alternating current feeds the motor. If you change the input voltage it will not change the speed, but may destroy the motor. If the motor does not have brushes, you cannot use a motor speed control.