Following the release of iOS 12.4.1 a couple of weeks ago, Apple has now stopped signing iOS 12.4. What this means is that it is no longer possible to downgrade or upgrade back to iOS 12.4.
Apple had accidentally unpatched a security exploit in iOS 12.4 which made it possible to jailbreak it. It was after years that the latest public release of an iOS build could be jailbroken.
If your iPhone or iPad was not already running iOS 12.4, there is now no way you can downgrade or upgrade to it unless you had installed it previously on your iPhone or iPad and saved the necessary SHSH blobs.
It is extremely unlikely that we are going to see an iOS 12.4.1 jailbreak. All the major developers in the jailbreaking community are currently working on adding A12 Bionic support to their iOS 12.4 jailbreak tool. Once they are done with that, they will turn their attention towards iOS 13 instead of focusing on iOS 12.4.1 especially since iOS 13 is now a week away from its public release.