Final answer:
In RESTful APIs, the HTTP method PUT is used to replace an existing item at the URI, differentiating it from POST for creation and PATCH for partial updates.
Step-by-step explanation:
To replace an existing item at the URI in a RESTful API, the HTTP method used is PUT.
This method is idempotent, meaning that making multiple identical requests will produce the same result as making a single request. In contrast to POST, which is used to create a new resource, and PATCH, which is used for partial updates, PUT is meant to update an existing resource or create it if it does not exist at the specified URI.