1. Click on the Website & Domains tab (if not already on that page).
2. Click on File Manager.
3. Click on the error_docs folder (the text link or the folder in front of the link).
4. Click on the error page you want to edit.  For example, Not_Found.html.
5. Use Plesk’s built-in HTML editor to edit the error page.
6. Click OK to save your changes.