How to update virus definition on Norton Bootable Recovery Tool

Hi,

I used to use 'Symantec Endpoint Recovery Tool' (SERT) to clean infected computer.  But since Symantec no longer support this anymore and tell us to use 'Norton Bootable Recovery Tool' (NBRT), I have not figure out a way to use NBRT with the latest virus definition.

I downloaded the latest NBRT on 21 July 2016, and its virus definition date is 26 May 2016, which is almost 2 months old.  I then contact Symantec support and they told me there is no way to update the definition like they did on SERT. 

Has anyone know a way to update the definition on NBRT?

Thanks!