The reply button is on the top of a ticket.
If you click on it, a new reply-box appears.
Tip: "Send and go to next" vs. "Send and stay"
If you click on the arrow beside the "Send" button you can choose between two behaviors or change your default.
Save time by using Quick-replies
Store and load your most common replies as quick-reply snippets. Also known as a canned response this feature will save you a ton of time.
Just click on this button at the right of the reply-box.