You have 3 options you can set for the submit success handler (Setting/Form Settings/Specific Form/Submit Success Handler):
1. Custom message displayed on the same page - CLICK HERE for more - Submit Success Handler Message
2. Redirect to another page
If you select the redirect option you can redirect to whatever thank you page you want so you have full control over it. If you choose to just display the thank you message on the same page you can customize that however you want.