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This paper proposes a method of nonmonotonic theory change. We rst introduce a new form of abduction that can account for observations in nonmonotonic situation. Then we provide a framework of autoepistemic update, which describes nonmonotonic theory change through the extended abductive framework. The proposed update semantics is fairly general and provides a uni ed framework for various update semantics such as rst-order update, view update of databases, and contradiction removal of nonmonotonic theories.