Your ticket is non-refundable, but possibly changeable with a fee. So you probably can change your return date by paying the change fee plus the fare difference. You just need to call the airline office where you bought the ticket from, presumably in India (since you said you will return to India).
Now, before you call them, there’s a bit of homework you can do. If your date is flexible, try to see which date is the cheapest. As I stated before, you will need to pay the change fee plus the “fare difference”. If you’re lucky to find the same fare on the new return date, you will only need to pay the change fee. On the other hand, if you pick a date with only expensive fares left, then you may pay a lot more than you would like.
Nonetheless, you will have to call the airline office or the agency in India to find out the exact amount.