AWS S3 with WWW Subdomain
If you don't have a www subdomain bucket on S3, then anyone who goes to your website with a www prefix will not be able to access the site.
Make a seperate bucket for your site with a www subdomain. Then right-click on the bucket and select it's properties. From there go to the Static Website Hosting tab, and select the Redirect all requests to another host name option. Make sure then that the correct S3 bucket you wish to point to is selected and save.
CNAME the WWW with your domain registrar
You'll have to go to your domain registrar and edit the DNS records. Create a new CNAME and point it to the bucket with the www subdomain.
As soon as the DNS records are updated, any request for your website with the www subdomain will redirect to your proper domain.