Eureka Forbes Aquaguard - defective product

I purchased an Aquaguard online on ** june ****. It was a partially defective product. The defect was found by the technician who installed it. He asked me for the invoice so that he could replace the product. I didn't have the invoice. So I requested the company for the invoice which was long delayed. On the pretext of delayed submission of the invoice, replacement of the product was denied. I pleaded the company to atleast replace the defective part. They are demanding me that I should pay for the part or else the defective part would not be replaced.