01 : Non Root Disk For the external volume which doesn’t have the operating system just detach the volume and extend it then reattach again.

02: Root Disk But for OS root disk you will need to take a snapshot from the volume and recreate the instances from the created snapshot as per below steps.

1. create a snapshot of the old instance volume.
2. create a new volume from the created snapshot with required disk space .
3. create a new instance from this new volume. Make sure to create the instance in the same volume availability zone.

If you want to use the same private and public IP addresses

4. Disassociate the floating ip from the old instance.
5. Detach the network interface card from both instances.
6. using OpenStack CLI run

#nova interface-attach --net-id <net-ID> --fixed-ip <old-Private-IP> <New-Instance-ID>

7. associate the old floating ip to the new instance.