[Plugin: Contact Form 7] How do I change my landing page?

right now, when someone submits a form on my “get a box” page at http://www.thelovequilt.com, they get taken to a landing page that says something along the lines of, “thank you for your submission..blah blah blah”. is it possible for me to add to that message? i want to add a link to that landing page that will lead to another page that i am currently creating. i had someone make and install this form and landing page, and they are not responding to my emails, so i am on my own. do i have to change a file in my host, or can i just do it in my back-end of my wordpress site? if i have to change a file, which file do i have to change?