You can use the Edit Security Group dialog to edit the description of a Security Group resource for the target AWS platform.
In the Web Interface toolbar, select the Platforms tab.
Select the platform, then click Edit.
Under Associations, select the Security Groups tab, then click Edit (or double-click the security group) to open the Edit Security Group dialog.
Modify the Description for the Security Group resource.
Click Save to update the Security Group resource.
Click Close to exit the dialog.