How expand lxd disk

hi.

I deployed Charmed Kubernetes #679 .

The deployed kubernetes-mastr node root-disk is 16G.

But now ,the disk is full.

the question is :How can scale the disk to 32G?

I used command pasted here:

lxc list

lxc config show cosmic-elf

  root:
    boot.priority: "0"
    path: /
    pool: default
    size: 60GB
    type: disk

but when I ssh this node , df -h, /dev/sda2 still 16G.

How can I do?

Thank you.