Clients can request a specific version in one of two ways:
Via the Zotero-API-Version HTTP header (Zotero-API-Version: 3)
Via the v query parameter (v=3)
Use of the Zotero-API-Version header is recommended for production code. The v parameter can be used for easier debugging and sharing of API requests or in clients that can't pass arbitrary HTTP headers.
The API version of a response will be returned in the Zotero-API-Version response header.