To edit the Redirect Thank You page there are two steps.
STEP 1 CREATE A THANK YOU PAGE
Search for an example Thank You page, by searching for 'Hidden' Page in the main pages area.
Create a new Thank You page, just like any other page, though remember to save your pages as 'Hidden' pages.
STEP 2 REDIRECT A FORM TO THE THANK YOU PAGE
In the Settings area of each Form you can select your Thank You page from the drop down list of pages.