Taken from : https://us.battle.net/support/en/article/15893

1. Download and install the Blizzard Battle.net app

2. Launch the app

3. Log in with your Blizzard Account

   1. If you do not have a Blizzard Account you can create one for free

4. Select the game you wish to install

5. Click the Install button to download and install the most recent version of the game