how can i change the landing page from thank you to another web page on the internet

If you wan the users to be redirected to another web-page instead of the Thank You Page, please check this guide: http://www.jotform.com/help/38-Redirecting-Users-to-a-Different-Page

If you want them to see a Thank You Page, and get them redirected to a specific web-page after, you will need to create a custom Thank You Page and create the auto-redirect, please check this guide: http://www.jotform.com/help/89-Create-a-Custom-Thank-you-page-with-Auto-redirect

Let us know if you need more help, we will be glad to assist you.