"You must create a DB snapshot before you can restore a DB instance from one. When you restore the DB instance, you provide the name of the DB snapshot to restore from, and then provide a name for the new DB instance that is created from the restore. You cannot restore from a DB snapshot to an existing DB instance; a new DB instance is created when you restore."
http://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/USER_R...
"You must create a DB snapshot before you can restore a DB instance from one. When you restore the DB instance, you provide the name of the DB snapshot to restore from, and then provide a name for the new DB instance that is created from the restore. You cannot restore from a DB snapshot to an existing DB instance; a new DB instance is created when you restore."