Does the Pope have the authority to overturn pronouncements of previous popes, for example, in matters of contraception and the ordination of women?

We must distinguish between different types of law and teaching. There are certain laws and precepts that are ceremonial and customary practices, or merely disciplinary rules. These can be changed, and are changed from time to time. For example, things such as the kinds and types of vestments and other regalia worn by the clergy and other merely ceremonial aspects of the liturgy can be changed. Disciplinary norms such as curial structures, canonical penalties, etc. can also be changed. 
 
But in matters of defined doctrine by the Magisterium regarding faith and morals, rooted in Scripture and Apostolic Tradition, or from previous Popes and Councils, the Pope is bound to uphold them. There are some technical debates about what is definitively taught that are too technical to set forth in this short answer. But the two matters, which you cite, are certainly teachings to which the Pope is bound and may not overturn.