Hello Ancelot,
Downloading versions of Wwise is done through the Wwise Launcher application. To download Wwise, the link here will take you to Audio Kinetic's download page for a MacOS version of Wwise Launcher. If that is not your client, there is a link on the site that can take you to the download page for a Windows user.
Once you've downloaded the application and opened it, or if you already had done so, any version of Wwise can be installed by clicking Wwise in the left-hand view. This should bring you to a page where you will be looking for the "Install New Versions Of Wwise" header. After choosing a version of Wwise, click Install at the bottom. If you're doing this for a certification, be sure to check what versions of Wwise and Unity it wants you to run, as this will determine what you want to be installing. If this is for a personal project and you plan to integrate a Wwise project into a game engine, be sure to check Audio Kinetic's Documentation Page. You can find info regarding what versions of Wwise are compatible with varying versions of a Game Engine.
I hope this helps!