How can I update a specific field of an existing document?

[Iris Soto]

Hello everybody,
I want update a specific field in a document, but i don't find how do it 
in the documentation of Solr.
Is that posible?, I need to index only a field for a document, Do i have 
to index all the document for this?
The problem is that i have to transform a bizdata object to a file 
content xml in java,  i should to build all the document xml step by 
step, field by field, retrieving all the bizdata of database to be 
passed to Solr.

Thanks in advance.

--
Iris Soto