Paytm Mall - wrong product delivered

I ordered sony external hard drive of * tb from paytm mall app on ** april ****. the order id is **********. for this i paid an amount of Rs.**** (rs.*** received as a cash back on dispatch of product).On ** april ****, they delivered the order to me, when i opened the packet i found **gb sd card of samsung make. i address the issue to delivery boy but he refused to entertain me as there work was to deliver the packet which he has completed and advised me to contact paytm mall customer care. quikly, I called customer care, after waiting more than ** mins. i was able to contact the customer care. however, they simply refused to take the complain citing the justification "product is non returnable. i asked them, i received the wrong product not the desired one. Therefore, you must register my complain. after this they disconnect the call.
they cheated me.
Please help me.