A 404 error means that the browser tried to access a page that the server couldn't find, so the easiest way to replace the error is to update the link to point to the correct page or to create a page for the link. Another option is to redirect 404 errors to a search engine or index that can help the user find the missing page.
One of the risks of redesigning a website is losing links. Search engines and other resources collect and index links, and many websites get the bulk of their traffic though search engines and links on larger sites. These broken links typically result in 404 error codes. Users who type in links manually might also encounter 404 pages if they make mistakes while typing.
Website administrators can check to see what pages users tried to access that resulted in 404 error codes. This information can help administrators fix broken links users encounter frequently. If a large amount of traffic comes from a small number of links on external site, it might be worth contacting those sites' administrators with updated links. Another option is to redirect users to an internal search engine to help them find the page's new address.