Taring, (or Zeroing) the HM-5 is not available using iOS devices (only the B-Ware Android app has this capability). The reason is that Apple has not opened their NFC implementation to writing data - data can only be read by iOS devices and taring necessarily involves writing a command to the monitor.
This feature will be added once Apple adds the ability to write to their iOS smartphone operating system.,
In order to tare an HM-5, please borrow an Android with NFC on, say an annual basis, to tare the hive monitor at the start of the season.