In the FAQ's it says that you have to do a manual incremental update:

How do I update a document or a set of documents that are already indexed?
>
> There is no direct update procedure in Lucene. To update an index
> incrementally you must first *delete* the documents that were updated, and
> *then re-add* them to the index.
>

How do I determine the existing document that matches the document I am
updating?