Yes, you'd better backup iPhone before repairing to prevent the data missing issue. Indeed, iCloud provides easy way to backup iPhone.Go to Settings>iCloud and then scroll down to Backup.Here you can turn backups on and initiate a new backup immediately. If this is your first iCloud backup it might take a while to complete. Once the backup is complete, though, your iPhone will automatically backup when idle for an extended period of time and on an active wi-fi network. Of course, it is also simple to restore from iCloud after resetting your phone and choose "backup from iCloud".