SECURITY GROUPS:
A security group acts as a virtual firewall that controls the traffic for one
or more instances.
When you launch an instance, you associate one or more security groups
with the instance.
You add rules to each security group that allow traffic to or from its
associated instances.
You can modify the rules for a security group at any time; the new rules
are automatically applied to all instances that are associated with the
security group.
Click on Security groups section under NETWORK & SECURITY from
the left pane.
CHANGING OR ADDING SECURITY GROUPS TO YOUR EXISTING INSTANCES:
Click the instance from Instance tab and select the instance, then click on Actions.
Click on Security and select Change security Groups
By Clicking on ADD SECURITY GROUP , You can change the security group
Click on save option to change and save your security group.