The LP and TY page are separate pages. Usually I try to setup the TY page first, because the LP Form needs a page to point to after the form fill out.
You can visit the LP any time and click on the Form, then press Edit on the gear box to change its settings.
To edit any page (approved or not) you can right click on the page or select then press Edit.
Remember to Approve any Page before placing the URL in an email or on another page. Otherwise the changes you made will NOT be seen.