iOS 12.4 is no longer being signed by Apple, which means that officially, there’s no way to downgrade to 12.4 once you’ve updated to iOS 12.4.1.
Apple has made it a practice to stop signing older software versions when newer ones are released to encourage users to keep their devices updated. In this case, 12.4.1 was quickly created due to a huge vulnerability discovered in iOS 12.4.
The bug allowed hackers to make up jailbreaks for devices that are running iOS 12.4 while leaving them vulnerable to hacks at the same time. Once Apple learned of the vulnerability, the company quickly released the iOS 12.4.1 as countermeasure. With the fix, jailbreaking will no longer be possible.
It’s worthy to note that 12.4.1 is available to both iPad and iPhone, but iOS 13 is expected to be released next week and is currently under final beta testing.