Well you can check the link above or even go to the Mi Service Centre in your city. I had an issue with my battery about a little over a year ago and when I had it replaced I think i paid somewhere around Rs. 1700 at that time for the battery replacement since the battery warranty had run out.

But be prepared to hear that the Service Center may not have the battery handy at that particular time which means they wont be able to instantly replace it. Usually they tell you to leave the phone with them for about 7 days max (which is the time period for any part to be shipped to them) for the replacement.