Try this in web.config file
<configuration>
<system.webserver>
<rewrite>
<rules>
<rule name="extensionless" stopprocessing="true">
<match url="(.*)\.html$" />
<action type="Redirect" url="{R:1}" redirecttype="Permanent" />
</rule>
<rule name="removeextension" enabled="true">
<match url=".*" negate="false" />
<conditions>
<add input="{REQUEST_FILENAME}" matchtype="IsFile" negate="true" />
<add input="{REQUEST_FILENAME}" matchtype="IsDirectory" negate="true" />
<add input="{URL}" pattern="(.*)\.(.*)" negate="true" />
</conditions>
<action type="Rewrite" url="{R:0}.html" />
</rule>
</rules>
</rewrite>
</system.webserver>
</configuration>