[doublepost=1473204477][/doublepost]I do not want to update the iOS. I want to maintain the iOS 8.1.2 I have.Connect iPhone to computer and open iTunes.
Backup to iTunes - assuming your phone is functional and you can do a backup.
Power phone off (leave connected to computer with iTunes running).
Put phone in DFU Mode (Google how).
When iTunes detects iPhone in recovery mode, Restore and Upgrade.
When iTunes is done and your phone is restored, restore from your iTunes backup to get your data back.
You will be losing your jailbreak and will be restored to 9.3.5. If that's not what you want then I can't help because I've never used Cydia Impactor.
Then, wherever your situation is you're stuck unless you can get Impactor to work.[doublepost=1473204477][/doublepost]I do not want to update the iOS. I want to maintain the iOS 8.1.2 I have.
Cyda Eraser is the new name for Cydia Impactor.Cydia Eraser is all you need, get it from Cydia, backup phone, turn on wifi, run Cydia eraser, let it do its thing, phone restarts you are back to fresh install as you would of got it from day one! This will remove your jailbreak and factory restore but you can re-jailbreak.
I did it on my 9.3.3 i6S and 6 ios 8.4!