Middle name & sur name problems

Good pm.pls help me w/ my prob.im 18 yo. I dont have middle name & im using my mother's surname and it also appears in my bcertificate.but im not an illegitimate child since my father signed an acknowledgement in my bcertificate and i was late registered.i use my mother's name as my lastname and also her middle as my m.name in my school records.my other siblings use my father's last name and my mother's as middle name.i want to change my last name to my father's last name.im in 3rd yr college now and i want to fix these things before taking the nsg board exam.my father is already dead. What should i do & what are the requiremnts to process this? Pls help me. Thanks & more power.