how do I change the thank you message?

Hello apevitz,

Yes it is possible to change the default thank you message. You can add your own thank you message or redirect users to your own thank you page. The following guide should help you:  http://www.jotform.com/help/211-Setting-Up-the-Thank-You-Page 

Hope this helps.

Do get back to us if you have any questions.

Thank you!