A 404 error shows when a page is not found - this can occur when a customer has manually mistyped a URL, or clicked on a link which does not lead anywhere on your site.
Links leading to 404 errors commonly appear when merchants send out links to a products or pages on social media, and then remove or alter their locations at a later date.
Within your Shopify admin, go to Sales Channels > Online Store > Themes.
Click your theme, and select Customize.
Go to the drop-down menu at the top of the page.
Scroll down to select 404 page.
To change the text on the 404 page, edit your language settings within your Shopify admin space.
Within your Shopify admin, go to Online Store > Themes.
Under Themes, click the Actions drop-down menu and select Edit languages.
Under 404, enter your new text.
Use the Title and Subtext html entry fields to choose new text for your 404 page.
These changes will then be reflected on your live store.