you change it in your settings for your form to the page you want people to return to after they submit a form to you

or is it for an e-mail list then there is a setting there to make a thank you page for people to go to after they subscribe and a goodbye page area for when they leave

so ONLY YOU can change where people go to and make sure you SAVE the changes you make then test it

You are using a hosted forms service so there is nothing you can do in the html code to change this. Log in to your account with your service provider and change the target page through your account settings.