Editing the 404 Error Page
From your store's Dashboard, click the Site Designer icon on the left Navigation menu.
Choose "Page Not Found" from the Page: Home dropdown menu at the top of the page.
To Edit the contents of the 404 Message Block hover of the area and click the Edit button that appears on the left side of the Block.
Click the Edit Text button to make changes to the Block's content. You may edit the text, add images, and create links using the Text Editor. Click the Update button when you have completed your changes.
Click the Done button to save you changes to the 404 Error page and if you are ready to apply your changes to your live site, also click the Publish button located at the top left of the Site Designer page.