First of all, you have to install and activate Redirection plugin on your site.
After activating the plugin, click on Tools >> Redirection to setup Redirects
Here you add your old URL (which URL is showing 404 error not found) in the Source URL field and add the new URL in the Target URL field where you want to redirect the user. After that leave the rest of the settings as default.
After adding your URL click on “Add redirect”. Now to test it, you can click on the URL with 404 not found.
Now when a visitor clicks on that 404 not found page it will automatically redirect them to the new URL.