I can't unlock my iPhone 6 - used touch id too many times and passcode is not being accepted

If Touch ID doesn't recognize your finger, you'll be asked to try again. After three attempts, you'll be given the option of entering your passcode. After two more tries, you'll need to enter your passcode. In addition, you'll need to enter your passcode to unlock your iPhone in these conditions:

• After restarting your iPhone
• When more than 48 hours have elapsed from the last time you unlocked your iPhone
• To enter the Touch ID & Passcode settings.

If the phone doesn't recognize your passcode you are not entering the correct passcode. You will have to restore the phone and set it up as new. See: iOS: Forgot passcode or device disabled

That is actually not correct, there is a clear bug that dates back to iOS6 and has been carried forward all the way to iOS8. Sometimes, upon updating the operative system, a newly established passcode or an old passcode (contained in a backup) might simply stop working (i have also seen a case where the passcode randomly worked once, only to stop working at the next unlock attempt). Not sure what it is but it happened to both devices bought at Apple AND devices that were sold as unlocked by a carrier. Furthermore iPhones do not seem to like locked SIM cards.

Anyway, turn iTunes on, switch your phone off, when the activity indicator has completely disappeared plug the cable leading to your computer, keep home pressed, follow the instructions to restore to factory settings. You can restore from an iCloud or an iTunes backup. If you have no back up, consider your content lost and start afresh.