Apple has released iOS 13.4.1. and iPadOS13.4.1 to fix the bugs in iOS and iPadOS 13.4 that was released two weeks ago.
iPhone and iPads get a fix for a bug that prevented devices running iOS and iPadOS 13.4 from making FaceTime calls with devices running iOS 9.3.6 and earlier or with Macs running OS X El Capitan 10.11.6 and earlier.
There's also a fix for a bug with the Settings app which prevented Bluetooth from being launched from the quick actions menu on the Home screen.
iPad Pro 12.9-inch (4th generation) and iPad Pro 11-inch (2nd generation) users get an additional fix in the form of a patch for a bug that prevented the flashlight from switching on after tapping the Flashlight button in Control Center or on the Lock screen.
UPDATE: These packages do no contain security fixes.
Updates can be downloaded over the air by going to Settings > General > Software Update. The download is around 100MB.