Milton — thermosteel flask 500 ml

I bought milton thermosteel flask in april 2018.. We are regularly using it since may. Now the flask is hot outside and the water cools down within 3-4 hrs.. I misplaced warranty card.. Kindly help me in this regard. We are regular customer of milton brand. We paid around 600 for this flask. Pls help us to replace the same. Pls take this issue to concerned person