You can edit the Thank You page of the form by following these steps:
1. Hover your cursor to small M ( on the top left ) and click on "Web".
2. Click on the Forms tab and search your form name from the Search bar.
3. Click on Edit under the name of the form.
4. Now you are on Step 1 of the form and you can edit the Thank You page on Step 4 so please click on Step 4.
5. After editing the Thank You page click on Save.