How to calibrate HR202 humidity sensor?
As WhatRoughBeast already mentioned the data sheet is readily available by simply goolgling "HR202 Humidity". Here is the link. The accuracy and resolution are clearly stated. 

Note that the sensor is analogue and requires an AC voltage! It is not a particularly user friendly Humidity sensor for this reason. It is a resistor which changes its resistance based on the humidity of the environment. 
How accurate your final humidity reading will be will also be a function of how accurately you can measure its resistance. 

On the datasheet there is a table which tabulates the resistance of the sensor at various Relative Humidity levels given a certain temperature. From this you can create a 2 parameter equation in software which takes the current temperature, and resistance value from the sensor as inputs and calculates your humidity value. 
Note, you will also need a temperature sensor to use this humidity sensor accurately. 

Actually Calibrating a Humidity Sensor:
Humidity Sensors are notoriously difficult to calibrate, because you need a known Relative Humidity value to calibrate against. The way this is usually done is using saturated salt solutions, which at equilibrium result in known relative humidities. Here is a paper tabulating the produced relative timidities at different temperatures: http://nvlpubs.nist.gov/nistpubs/jres/81A/jresv81An1p89_A1b.pdf 

From these values you can calibrate your sensor if need be.