Before increasing the size of your volume you should take a snapshot of your volume. Be sure to power down your droplet before taking the snapshot to ensure data consistency. You do not want something writing to the drive (Or syncing in the case of Nextcloud)
Research
How to mount external storage to Digital Ocean
You can also mount other shares using the occ command implemented in Nextcloud 9:
https://docs.nextcloud.com/server/9/admin_manual/configuration_server/occ_command.html#files-external-label.
This guide shows how to see the file system of your Nextcloud (probably the most helpful I have seen so far: https://help.nextcloud.com/t/unable-to-use-entirety-of-hard-drive-for-storage/52823/8