Using blow code you can remove all .html Just add this code in your root location .htaccess

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RewriteEngine on

RewriteCond %{THE_REQUEST} /([^.]+)\.html [NC]
RewriteRule ^ /%1 [NC,L,R]

RewriteCond %{REQUEST_FILENAME}.html -f
RewriteRule ^ %{REQUEST_URI}.html [NC,L]

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old URL yourwebsite.com/about-us.html
New URL yourwebsite.com/about-us