Currently, restoring data is limited to what's stored in the cloud (ie Google Drive, iCloud Drive), or what can be manually backed up off the device.
Instructions on how to back up data can be found here; once backed up, data can be restored by:
- Re-enabling iCloud Drive for the apps that support it.
- Re-importing previous data that was backed up manually
The first is accomplished by re-enabling iCloud Drive in Settings→[Account at Top of List]→iCloud and then re-enabling syncing in the apps themselves. During the first launch of the app, it should display a prompt to enable iCloud syncing if it supports it
The second is accomplished by manually re-importing the previously backed-up data, usually on a per-document, per-app basis.
The example below outlines the process to restore manually backed-up data to Book Creator, but most other apps that tie into a cloud service will follow the same pattern:
1. In Book Creator, tapping the "+" pulls up an options menu, with an option to import a book from an external location
2. Tapping the option to import then allows us to pick the location, in this case, iCloud Drive is the main option; selecting that presents a window displaying saved files in iCloud Drive where we can then restore from
Once the data has been selected for a restore, it will download from iCloud (in this case) and be accessible on the device. Some apps support restoring multiple files at once, but most require files to be restored one at a time.