What is the procedure to change my son's last name ?

Sir/Madam My first husband was died four years before and I have one son. My son's age is 6 years. I got second marriage in the month of June 2016. My son is studying first standard and his name is registered in records using my first husband name because his birth certificate has my first husband’s name as father’s name. Now I want to change my son’s initial (last name) using my second husband name. Legally is it possible? Also I have assets on my first husband name. I belong to Hindu religion.If the initial change is possible, will there be any problem come in selling of the assets ?