Navigate to the Steps tab and then click the Confirmation step button. Write your Confirmation/Thank you text and click the yellow Save button to save you changes.
If the Confirmation step is set to disabled when you want to edit its content, you will be alerted.
Press the link in the yellow alert message Click here to enable to enable the Confirmation step again directly from the Steps tab, as shown in the picture below.
It is possible to write simpler HTML with the smart WYSIWYG editor for the confirmation/thank you text.