Procedure to transfer deceased father's property to mother

Hi, My Father has gone to the heavenly abode. But we have a home in co-op housing society and we want to transfer the property in my mother's name. We have submitted all the relevant docs to the chairman and secretary of the society. But now he is saying that the relevant papers required for transferring property in my Mother's name are lost. So what is the legal procedure to transfer the property in my Mother's name? My Father had left a will to transfer the property in my Mother's name.