Variante 1 (Restore a full backup):
If you have a full backup of your iPad/iPhone that still had the old version of SongBook+ installed, you can reset the iPad/iPad and restore it from this full backup.
Variante 2 (Restore a previously saved .ipa file):
Since you can no longer manage the installed apps on a device (iPad, iPhone, ...) with iTunes, Apple unfortunately no longer offers the option of downgrading to an older version of an already installed app.
Before that, you still had the chance to find an old version of the app (in the form of an .ipa file) in the recycle bin on your computer (PC or Mac) and transfer it back to the device using iTunes. But only if the device was already synchronized with the computer via iTunes.
But there is still software available from other vendors with which this is still possible. The prerequisite is that you still have the .ipa file of the old app version. In other words, that you used the software before the update to back up the .ipa file of this old version. You can then delete the current app version on the device and reinstall the saved, old app version.
One software that makes this very easy is "iMaziing" (https://imazing.com/en).
Backing up and restoring apps is already possible in the free version.