This is a new feature with version 2.4
To export as ICS open the 3-dots-menu (⋮) in any calendar view and select print/export, then select ICS.
Exporting as ICS is currently possible as the whole calendar only.
ICS export is not yet recommended for backup purposes, as exporting and subsequently importing does not reconstruct your data 100%. We strongly recommend syncing with a cloud service to keep your data safe! If you prefer using local data, make sure to have a proper backup solution working.