https://example.com secure but https://www.example.com not secure

When users go to my webpage, they type in www.example.com, this then takes them to https://www.example.com, which is not secure.

When a user types example.com, this then takes them to https://example.com, which is secure.

How to forward www.example.com to example.com so that the user always gets redirected to the secure https://example.com ?