Attach the volume to the existing instance by following these steps:
- Stop Your Instance.
- Create a snapshot of the root volume.
- Create a new volume using the snapshot. [In case of ubuntu name it /dev/sda1]
- Detach Amazon EBS old root volume from already stopped instance, by right clicking on old EBS volume.
- Reattach the new Amazon EBS volumes to the instance.
- Start your Instance.