Hi Nabil,
Vectara API at the moment doesn’t support updating an existing document. You will have to call delete first to delete the existing document. After that, you can index the document again with updated contents.
Please note that a document is uniquely identified by it’s document_id
See here for details: Deleting Documents | Vectara Docs
Here is a similar post addressing this question: What is the recommended way to update a document?