The Pope who is the head of the Vatican and bishop of Rome is the world leader of the Catholic Church.
Due to such great position of authority as leader of the Church, many are of the assumption that the Pope has the power to change the doctrines of the Church. Well, to set the record straight, the Pope has no power to change doctrines. He is merely a custodian and keeper of the Church doctrines therefore, he has the responsibility to promote the teachings and doctrines of the Church in the best way possible without any alteration. This, he does with the guidance of the Holy Spirit.
The Church has its customs, cultures and ways of practice. The Pope reserves the right to change certain customs for the good of the Church. For example, for some reason, hopefully a good reason, he could decide that vestments of priests be made different from the ones that are being used to celebrate masses presently. By doing so he has altered the official dressing of priests, he altered the way of practice. While with regards to doctrines, he has no power to change them. For instance, the Church believes in the Holy Trinity; The Pope therefore does not have the right to alter such belief by saying that the Father, the Son and the Holy Spirit are not One God but Three entirely different beings; that would be downright awful as these doctrines form the basis of our very existence as Christians.
The Pope can make some minor changes to certain ecclesiastical laws that guide the conducts of the religious and lay faithfuls, not the beliefs and Creed of the Church (doctrines).