Change in year of birth..

Hello, My uncle is 64 yr old. He had purchased a flat in co-op hsg society in 1990. It was a second transaction. We had an agreement deed with his signature on papers. Later he left India and not in touch anymore.. this property is in Nasik.. The property stamp duty was not paid, due to my uncle's unawareness. Then Thier children grew up and they inquired. Now they want to pay stamp duty and do the registration. Kindly advise on the procedure.. The seller in not traceable.. he no more stays at the address he had given us.