Back panel replacements

You need to contact an Apple Authorized Service Provider (AASP) in India, as I don’t think Apple has actually opened any stores yet in India (some are planned, however). You can go here to find the nearest AASP—>https://locate.apple.com/in/en/

It may also be possible just to replace that lens cover, so don’t give up hope. And possibly an independent shop could do it.

The back panel can’t be replaced; it is the substrate that all of the innards are glued to. You need a replacement phone. Here is the pricing—>iPhone Repair - Official Apple Support