How to calibrate Unipolar ADC in software
What exacltly is use of above ADC error data from manufacturer to end
  user ?


A zero offset error of +/-0.25% in a full scale of (say) 2.5 volts means that  a digital value of zero may be produced even when the input voltage is as high as +6.25 mV above 0V. 

This tells me that I can't rely on the ADC converting accurately below this value.

The gain error of +/- 0.25% in a full scale of (say) 2.5 volts means, I can't rely on using the top 6.25 mV of the ADCs input range.

This means I have to restrict my signal to a range slightly less than 2.5 V.


  Can we use these above data from manufacturer to calibrate my ADC in
  firmware ?


No you can't - you have to do it as a physical test for each ADC.