We have Nextcloud as a service. The service provider manages the Nextcloud application installation and upgrading as well as the OS. We have no access to the host on which Nextcloud is installed. The service provider performs daily system backups and keeps a limited history of them. This provides a solution to the problem of a restoring from catastrophic system failure but not of being able to selectively restore a changed or deleted file. Are there solutions that exist that would do an incremental back up on the data in Nextcloud to an external system, that would provide some sort of UI that would allow the selective restore of a deleted or an earlier version changed file?
hi @Rod_Oliver welcome to the community 
there is a Backup app. I didn’t test it but at least it claims single file restore is possible.
In general Nextcloud has multiple built-in functionalities which allow to take back previous state without restoring backup - all files keep some versions so the users can recover earlier state himself and there is a trashbin as well - so files remain on the system for some time by default.