If you want to jailbreak your iPhone but you accidentally updated to iOS 12.4, you can do that as of now. The iOS 12.4 ipsw file is still being signed by Apple for the time-being. This depends on what time you are reading this article as Apple may stop signing the software while you are reading this article. So before downgrading back to iOS 12.4 from iOS 12.4.1, go to http://www.ipsw.me and select your device type and see whether it’s still being signed. If it’s signed, you will see a green tick beside it and if it’s not, you will see a red Cross beside it. If it’s still being signed, immediately download the ipsw file for iOS 12.4 if you want to jailbreak. Do not downgrade to a non-signed version of iOS as it may Brick your device. Only do this if the software is still being signed.
After you have downloaded the ipsw file,
1. Connect your device to a computer, Take a backup of your iPhone because you will lose data.
2. If you are running MacOS Catalina, open finder and go to your iOS Device tab, and click of restore iPhone while holding the Option key. This will bring up the finder, from which you can select the ipsw file you just downloaded.
If you are running MacOS Mojave or below, simply open iTunes and click of restore iPhone while holding the Option key. This will bring up the finder, from which you can select the ipsw file you just downloaded. Make sure you are using the latest version of iTunes. On Windows PCs, you can press on the Shift key instead of option which will do the same action.
Make sure to restore back to a signed ipsw file only. Unsigned ipsw files may brick your device .
Downgrade at your own risk.