Apple releases new iOS firmware that makes it temporarily impossible to take advantage of unauthorised features available through jailbreaking your device. You may need to revert to an older version to jailbreak again.
Backup your device. Having a backup will help minimize any downtime if the downgrade process doesn't go well. See this guide for instructions on how to backup your iPhone.
Download the 7.1.2 IPSW file. An IPSW file is the firmware file that contains the iOS system software. You will need to download the signed IPSW file for 7.1.2 that is specific to your device. There are different files for different phones and different carriers. You can find the IPSW files at places like iDownloadBlog.com
Connect your iPhone to your computer. Open iTunes if it doesn't open automatically.
Select your device and click the Summary tab.
Load the IPSW file. Hold down ⌥ Opt (Mac) or ⇧ Shift (Windows) and click Update. Browse for the IPSW file that you downloaded.
Start the downgrade process. Click Update again. The downgrade process will begin. If this fails, you may have downloaded the wrong file, or Apple may have stopped signing the files digitally. If Apple is no longer signing the files, then downgrading will no longer be possible. Apple signs files digitally for a short time after the update is released, but they do not announce when they will stop signing them.
Setup your device. After downgrading, your iDevice will show the Setup screen. Follow the prompts to setup the device.