How to become a legal father for my step child. For her future education , passport , etc.,

I married a lady who already got married. For me this marriage is first and for her this marriage is second. Through the first marriage my wife got a girl child and her age is presently 5 years old. Her first husband expired after her first childs birth. Her first marrige is registered and the first marriage certificate is available. Her first husband is expired and his death certificate is available. The childs birth certificate bearing the biological fathers name is available. My marriage certificate with her is registered and registration certificate is available. Even though i have been called as a step father , i would like to adopt the girl child so that she becomes my legal child also. The reasons are i should be called as her father and she should get a good education, passport, share in my property and i have to take care of the child in all aspects and also the biological mother who is my wife. In her birth certificate and passport she should have my name as father. kindly guide and advise me the legal things if I have to do. I live in Mumbai. Thanks,