There are a few ways you can do this but if you have the URL Rewrite Module installed, it's fairly easy and a good way to do it.
You can paste the below configuration into your site's web.config file (enclosed in the <system.webServer></system.webServer>
section)
<configuration>
<system.webServer>
<rewrite>
<rules>
<rule name="https redirect">
<match url="(.*)" ignoreCase="false" />
<conditions>
<add input="{HTTPS}" pattern="off" ignoreCase="false" />
</conditions>
<action type="Redirect" redirectType="Found" url="https://{HTTP_HOST}{REQUEST_URI}" />
</rule>
</rules>
</rewrite>
</system.webServer>
</configuration>
OR you can use the IIS's UI to make a new rule like in the below screenshot.