I tapped Share in the History section, but wasn't sure which program to pick, as I didn't know what sort of file it would generate. I picked a notepad application, but I accidentally double-tapped, which set that to be the "always use" choice. Now the Victron app always tries to use the notepad app, and I can't change it. (The notepad app simply freezes when this happens; I guess it can't handle the file that Victron's generating)
I didn't see anything in the app's options to change the default program.
I've done Clear Cache + Clear Data for the Victron app, and also completely uninstalled and reinstalled it, but this program/file default association persisted.