Update existing files via Python SDK

Hi,

I need to update a file using the Python SDK. 

I know if a file exists and can get it's ID but can't find which Python SDK method to use. 

I've seen references to client.folder(box_folder_id).upload(WORK_PATH+item_file) and

chunked_uploader = client.file(item.id).get_chunked_uploader(item_file)
    uploaded_file = chunked_uploader.start()

But both throw this error: 

boxsdk.exception.BoxAPIException: Message: Item with the same name already exists
Status: 409
Code: item_name_in_use

Any suggestions?