Dunno what version of iOS you have but
Go to your HomeScreen
Tap "Files", a blue folder icon
You may be asked to Browse a list of locations
Tap "On My iPhone"
You should see a selection of folders, each with an App icon emblazoned on it and a text label underneath each folder.
e.g. "Agenda", "GarageBand for iOS", "Soulver", etc, etc
Find your BOI folder and tap on it.
It should show you a list of documents/files unique to your BOI app.
Select what you don't require and bin them.
This might be helpful
It's always useful to include details about the iPhone model and/or the iOS version installed in calls for help, otherwise we're back to "I've been told the discs on my BMW are too narrow" type inquiries.