I always save my steps in a text editor, here's what I put but you can probably find AWS docs super easily by Googling your question. Probably the same thing.
Run the following command replacing the values wrapped in carrots with your information:
aws elbv2 set-ip-address-type --load-balancer-arn arn:aws:elasticloadbalancing:<us-east-example>:<some-numbers>:loadbalancer/app/the-load-balancer/<random-string> --ip-address-type dualstack
Now update the dns record for the load balancer
With this IPv4 requests will resolve A, and IPv6 will resolve AAAA.
Dualstack Requirements (from AWS)
You can set the IP address type when you create the load balancer and update it at any time. Note that an existing internal Application Load Balancer can't be updated to dualstack mode. You must recreate the Application Load Balancer and configure it as dualstack.
VPCs and subnets that you specify for the load balancer must have associated IPv6 CIDR blocks.
The route tables for the load balancer subnets must route IPv6 traffic.
The security groups for the load balancer must allow IPv6 traffic.
The network ACLs for the load balancer subnets must allow IPv6 traffic.
Associate an IPv6 CIDR block with a VPC
aws ec2 associate-vpc-cidr-block \
--ipv6-cidr-block-network-border-group <region> \