This is really good guide:
http://cloud.tekgoblin.com/2013/04/29/aws-guides-how-to-increase-your-ec2-linux-root-volume-size/
basically stop your instance, detach the primary volume (/dev/sda1), create a snapshot from this ebs volume, create a volume (with increased diskspace) from the snapshot, attach the new bigger volume to the ec2 instance, as /dev/sda1.
start the instance again.