Backups are automatically created when your data changes (once for the day Outbank was opened).
If you want to manually create a backup, please follow these steps:
Menu > Outbank > Settings > General > Create backup
The backups will be saved under ~/Library/Containers/com.stoegerit.outbank.osx/Data/Documents/backups/
To open the backup folder click on 'File' > 'Backups' or go to the Settings of Outbank and select 'General' > 'Show backups'.