what do you do if you forgot your touch ID passcode

You have to enter in your passcode to turn off touchID, it wont accept your fingerprint. The only option would be to restore the device and you would loose all the data on the phone. You wont be able to do a backup, because once you restore your phone from the backup, the existing passcode will restore on the phone. so your only option would be to continue using your fingerpint to unlock device or Retore the phone and setup as new.