1. Locate the 404 error. Typically when there is a 404 error message on your website you are not aware of it. An easy way to locate 404 error messages is to sign up for Google Webmaster tools. Get a Google account and register your websites with them. You will have to insert a unique piece of code exactly where they tell you to to validate your site with them. Once that happens Google will regularly look at your sites and tell you if they find any 404 error messages. Alternately, you can ask users to contact you if they come across 404 errors on your site.
2. Determine why there is a 404 error. Once you have located the 404 error you need to determine what caused it. Typically this occurs due to a webpage getting moved or a webpage getting renamed. To see which is the culprit, look at the web address where the 404 error is and compare it to what the web address should be. That will tell you if the problem is a page moving or a page being renamed that is the problem.

3. Change the URL if the page was renamed. If your page was renamed, you will need to change the URL, or web address, of all of the links that were pointing to the old name. You will need to do this on your web page files that you should have stored someplace. Use a web design software, such as Dreamweaver or Frontpage, to make the changes. Then upload the newly changed pages to your hosting company with your normal FTP program to replace the pages that have outdated info.
4. Redirect the page if it has moved. If your webpage was moved then you will need to do a 301 redirect to fix the 404 error. You will need to create a .htaccess file, which you can do with Notepad or with the freeware Notepad program. You will have one line in your file. It will read: 'redirect 301 OLDLOCATION.htm http://www.NEWLOCATION.com'. Be sure to put the actual old location where 'OLDLOCATION.htm' is and put the new location of the page where 'http://www.NEWLOCATION.com' is. Save this file and upload it using your normal FTP program to your domain.
5. Test to see whether the error was resolved. Once you correct the problem that caused the 404 error, you should go to your website to test it out and see if it worked. You may have to click 'refresh' or F5 to see the most updated version of your website.