in web.config add this code and recycle application pool of your site and restart your site in IIS
<configuration>
<system.webServer>
<rewrite>
<rules>
<rule name="HTTPS force" enabled="true" stopProcessing="true">
<match url="(.*)" />
<conditions>
<add input="{HTTPS}" pattern="^OFF$" />
</conditions>
<action type="Redirect" url="https://{HTTP_HOST}{REQUEST_URI}" redirectType="Permanent" />
</rule>
</rules>
</rewrite>
</system.webServer>
</configuration>