Amazon EC2 Auto Scaling now lets you automatically update the instances in your Auto Scaling Groups (ASG) to release new application versions or make infrastructure changes. Traditionally, customers had to write custom scripts and build systems to update ASG configurations with new AMIs. With the new Instance Refresh feature, customers can now trigger configuration updates such as moving to new AMIs and changing instance types. You can roll out these updates to the ASG by replacing instances all at once or gradually.
Source:: Amazon AWS