Hello
Thank you for your question on PrestaShop thank you pages. Could you be more specific about the page? There are also modules that have their own Thank You pages. If you could describe when the page pops up, perhaps we could find which page that is and locate its content. As a general answer, you may want to check in the ps_cms table and see if the content is there. That can be edited via phpMyAdmin.
Kindest Regards,
Scott M
There is no "thank you page" in prestashop. The file that you are looking for is called "order-confirmation.tpl" and is located in ROOT > THEMES > YOURTHEME > ORDER-CONFIRMATION.TPL
Good luck