1) Head to the Heroes of the Storm tab on your Blizzard launcher.
2) Use the Region selector found above the "Play" button to switch to "Heroes 2.0 Beta."
3) Hit the big, blue "Install" button, and then log in to start playtesting!
There are also a few important details that you should be aware of during the Heroes 2.0 Beta:
Real money cannot be used to buy Gems or make any other purchases in the Heroes 2.0 Beta.
Instead, you'll earn Gems by completing a Weekly Quest that will only be available during the Beta.
Finally, any items unlocked or purchased during the Beta will not transfer to your live Heroes of the Storm account.