Update a document onto existing google document using Zend framework

Question

Hi guys I'm dabbling in google docs with Zend GData library - and succeeded to upload documents to google docs. However I would like to know how would it be possibel for me to upload a document and overwrite the document on google docs? Assume that I just have the docid which refers to the document on google docs.

Thanks again - I'm using Php and the Zend Gdata libraries