How do I clear caches and reinstall asset store files

iOS Pro Version I upgraded unity from the latest 5.0.x to the latest 5.1.x. In the process my project prefab models were broken. The models are all from the store. When I delete and reimport the models from the store they are still broken. When I create a new project and import the models they are fine. The models have all the pieces but they are not connected correctly(all pieces are spread out). How do I cleanly delete the imported files so I can install them cleanly(like a new project).