I jailbroke my iPhone 6 with iOS 8.1.4. If I update to iOS 8.4, will it crash, break the jailbreak, or hurt my iPhone?
With any Jailbroken device, any update of any kind will break the Jailbreak. OTA updates (update from device in settings app) will kill your phone and force a restore. Updating through iTunes will revert the Jailbreak but leave the rest of your data untouched. I recommend updating to 8.4 and jailbreaking again with the new TAiG jailbreak tool (plus it gives you some laughs during the process because the translation is so horrible)!
Hope this helped, Nick
It depends how you update it. If you connect to iTunes and update you shouldn't have problems, however, if you use OTA (the button in settings) your phone will be "bricked"
update with itunes to the ios 8.4,this is the best method . you can jailbreak again with TaiG 2.4.1 read more http://rootjailbreak.com