Is there a human involved in updating/replacing the document, or are you looking to fully automate the process with the API? As Ergin stated, it's not possible to add/remove documents using the API once an Envelope has been Sent. However, it IS possible for the Sender of the envelope to "Correct" (and re-send) the Envelope using the DocuSign web console -- including adding or removing documents for an Envelope that's already been Sent, but documents can be ONLY be removed/replaced IF none of the recipients have yet acted on the Envelope. i.e., once at least one recipient has acted on the Envelope, it won't be possible for the Sender to remove/replace documents in the Envelope (although they can Add new documents, as long as the Envelope is still in-process).
You can use the API to launch the "Correct Envelope" view as follows:
POST https://demo.docusign.net/restapi/v2/accounts/{acctId}/envelopes/{envelopeId}/views/correct
The response to this operation will return a URL that you can use to launch the "Correct Envelope" view.
Finally, one important caveat about using the "Correct Envelope" feature to enable the Sender to modify documents in an envelope: once the person is logged into the DocuSign web console where they'll correct the envelope, they'll have full access to that user account in the DocuSign web console -- i.e, they'll be able to access all Envelopes in that user account (Sent Items, Inbox, Deleted Items, etc.), send new Envelopes, access user settings via "Preferences", etc. So, be careful using the Correct view if you're concerned at all with providing the person with full access to that user account within the DocuSign web console.
Note: In order for the Sender to be able to add/remove Documents in the "Correct Envelope" view, the "Advanced Correct" feature must be enabled for the DocuSign account (in Preferences >> Features).