As you've already tested HTTPS working fine when your website is visited with HTTPS already. If it is not happening, then you can add this file .ebextensions/loadbalancer-terminatehttps.config with content as below:
option_settings:
  aws:elb:listener:443:
    ListenerProtocol: HTTPS
    SSLCertificateId: arn:aws:acm:us-west-2:<your-account-id>:certificate/<certificate-arn-on-aws-acm>
    InstancePort: 80
    InstanceProtocol: HTTP
Whatsoever left is to configure the instances Apache config to redirect the clients visiting your website with HTTP to HTTPS, add the code below to a new file .ebextensions/001_ssl_rewrite.config
For Apache 2.4+
files:
    "/etc/httpd/conf.d/ssl_rewrite.conf":
        mode: "000644"
        owner: root
        group: root
        content: |
            RewriteEngine On
            <If "-n '%{HTTP:X-Forwarded-Proto}' && %{HTTP:X-Forwarded-Proto} != 'https'">
            RewriteRule (.*) https://%{HTTP_HOST}%{REQUEST_URI} [R,L]
            </If>
For Apache 2.2.X
files:
    "/etc/httpd/conf.d/ssl_rewrite.conf":
        mode: "000644"
        owner: root
        group: root
        content: |
            LoadModule rewrite_module modules/mod_rewrite.so
            RewriteEngine On
            # This will enable the Rewrite capabilities
            RewriteCond %{HTTPS} !=on
            # This checks to make sure the connection is not already HTTPS
            RewriteRule ^/?(.*) https://%{SERVER_NAME}/$1 [R,L]
You can check which Apache is installed on your Elastic Beanstalk from these documents here