We study the notion of update of an ontology expressed as a Description Logic knowledge base. Such a knowledge base is constituted by two components, called TBox and ABox. The former expresses general knowledge about the concepts and their relationships, whereas the latter describes the state of affairs regarding the instances of concepts. We investigate the case where the update affects only the instance level of the ontology, i.e., the ABox. Building on classical approaches on knowledge base update, our first contribution is to provide a general semantics for instance level update in Description Logics. We then focus on DL-Lite, a specific Description Logic where the basic reasoning tasks are computationally tractable. We show that DL-Lite is closed with respect to instance level update, in the sense that the result of an update is always expressible as a new DL-Lite ABox. Finally we provide an algorithm that computes the result of an update in DL-Lite, and we show that it runs in polynomial time with respect to the size of both the original knowledge base and the update formula.