Here’s how to downgrade iOS 113 to iOS 11.2.6 or below on your iPhone/iPad using iTunes via IPSW file.
Note: If you are on lower version of iOS 11 like iOS 11.0.1, 11.0.2, 11.0.3, 11.1, 11.1.1 and 11.1.2, 11.2, 11.2.1 do not upgrade it to iOS 11.3 since you are on the right path for iOS 11 jailbreak and if you have already installed iOS 11.3, then you can downgrade it to lower version like 11.2.6.
There may lots of reason for not satisfying after updating your iPhone to iOS 11.3. So before it’s too late, downgrade your iPhone or iPad on iOS 11.3 to iOS 11.2.6 or lower and wait for a jailbreak release.
How to downgrade iOS 11.3 to iOS 11.2.6 or below
Since Apple is still signing the iOS 11.2.6, and downgrading iOS 11.3 to lower version is possible. So, follow this guide to downgrade iOS 11.3 to iOS 11.2.6 on iPhone/iPad.
Step 1# You need to upgrade or download iTunes to the latest version on your Mac or PC.
Step 2# Now Turn off Find my iPhone & disable passcode lock via the Settings app. (Settings > iCloud > Find my iPhone).
Step 3# Launch iTunes & connect your iPhone, iPad & iPod Touch using data cable to your computer.
Step 4# Hold the Alt/Option key on the Mac or Shift Key in Windows on your keyboard and click Update or Restore.
Note: IF YOU WANT TO FRESH INSTALL THEN GO WITH RESTORE OTHERWISE UPDATE IT.
Step 5# Now Select the signed ipsw iOS 11.2.6 file you had downloaded earlier
Step 6# Then click on the Restore iPhone.
Step 7# iTunes will inform that it will restore your iPhone to iOS 11.2.6 and Click Restore
After restored, your device will reboot to the welcome screen and will now install iOS 11.2.6 on your device.
That’s it. Your device should now be successfully downgraded to iOS 11.2.6 from iOS 11.3. Your downgrading process is completed; you need to now set up your device by following the on-screen instructions.