Many site owners receive a 404 Page Not Found error from Web Master Tools after changing their page link structure or theme over time. As someone who has fallen into this situation, I wanted to share with you a piece of information that I believe I found as a result of my research. As it is known, since the increase and non-resolution of these errors does not please Google very much, it is important to fix 404 errors.
Another method used to fix these errors is redirecting faulty pages to new pages using the 301 method via the .htaccess file. On different sites, this situation is described as the process of redirecting Site A to Site B, rather than fixing faulty pages. I couldn’t find much information about fixing faulty pages. As a result of my research and experiments, I learned that people who do not know how to create an .htaccess file need to be careful when editing this file and keep their backups 
A wrong code written into the file causes our site to crash completely and many pages not to open.
While I was in a state where I couldn’t find a way out, a plugin released for WordPress sites came to my rescue. With this plugin, which is very simple to use, it is now very easy to redirect all 404 error pages to a working page of our choice using the 301 method.
The name of the plugin: WordPress Redirection plugin - and the author is: John Godley
You can reach the plugin address Here.
You can download the plugin to your computer and upload it to your account via FTP, or you can install it by searching in the plugins section of your Panel. After downloading the WordPress Redirection plugin, activate it and it will take its place under the tools menu. When we open the plugin from here, the following text appears
Add new redirection
Source url : Enter the URL where you received the 404 not found error in this field
Match : url only Action: Redirect to url Regular expression (Do not change these settings)
Target url : In this field, you enter the address to which the faulty page written above will be redirected.
Then, by clicking the Add redirection button at the bottom, you redirect the faulty page to the address you specified. This process is a definite solution for those who do not have the knowledge to edit the .htaccess file. Thus, you regain all your faulty pages and all the backlinks you received before the page structure change in the past.
Edit:
You want to use the plugin I wrote about above, but you couldn’t access the content of your old pages at all. If you got a backlink related to that page in time, what should you do when you cannot access your old page?
When the page structure of your old posts is listed as /Urunler.aspx?uID=221, we cannot understand which page this page points to. Therefore, by scanning your page /Urunler.aspx?uID=221 via the web archive page, you see which product page of yours your old extension page went to. The web.archive.org/ site provides us with great benefit in this regard.
You can find the 404 unreachable errors you received from Web Master Tools one by one by searching on this page, and thanks to the plugin you installed on your site, you can redirect these errors to your new pages again with the 301 method with zero error.
As for old pages that cannot be found in the Web Archive search, you can redirect them to your homepage.