Unlocked Iphone from US not getting Applt support in India

I have an unlocked Iphone 5S which I had purchased in US and currently using in India. I want to change the battery of it at my expense, but the local apple support centers are not helping. This is really awful as I really need a new battery as the current one is draining out in very quickly. Need to know from Apple why is it so and how can resolve this with Apple support team.