I assume you're referring to Siri reading a text message and then replying. You can make make changes to your reply before sending. Once you've composed your message, Siri will tell you its ready to send and awaiting confirmation. At this point you can say "read my message" and Siri will read the message you just composed. Siri will then give you a bunch of options, "would you like to send, cancel, review, or change it." Never tried review, so not sure what that does.