If your site uses links like http://domain.com/testpage.html and http://domain.com/demo.html, and you want them to open like http://domain.com/testpage and http://domain.com/demo, you can use this solution.
Go through FTP or a File Manager to the folder with the site files. Open file .htaccess and add the following code:
RewriteEngine on RewriteBase / RewriteCond %{THE_REQUEST} ^[A-Z]{3,9}\ /([^.]+)\.html\ HTTP RewriteRule ^([^.]+)\.html$ domain.com/$1 [R=301,L] RewriteCond %{REQUEST_URI} !(\.[^./]+)$ RewriteCond %{REQUEST_fileNAME} !-d RewriteCond %{REQUEST_fileNAME} !-f RewriteRule (.*) /$1.html
where instead of domain.com use your domain name.
If file .htaccess (don’t forget to put a dot before the file name!) does not exist on the server, you can create it and paste the code from the example above with editing the domain name.