How do I insert a canned response into a reply?

In the actions row under the reply editor, click the "Canned responses" button (hover it to see the name) and pick a response from the list. The list shows the responses available for the ticket's group, sorted by name. The response replaces the content of your reply, attachments included, and you can edit everything before you click "Send". If you had already written something or restored a draft, Deskhero asks for a confirmation first.