I created two subnets without any preference of the availability zone(AZ). So, now I have two subnets in one AZ. I know that I can extend the VPC and create new subnets but I simply want to move a subnet into a different zone. Is there any way I can move a subnet to a different AZ on AWS?
This is not possible.
An Availability Zone is nominated when the Subnet is created. It is not possible to change the AZ on a Subnet.
You should either create another subnet, or delete the subnet and recreate it in a different AZ.