Like in every investigation, the how will come out at the conclusion of the investigations by the house. senate and special counsel.  Revealing it beforehand would compromise these investigations.  

Also, the FBI agents were satified that they received all of the details required to confirm how the Russians hacked into the DNC.  The exact how and answer to your questions I'm sure will be shared under oath in the classified session to the bi partisan committee, who will determine if the FBI's conclusion was just or not.