Assuming that you've created an ExitGames account (as detailed in the last chapter), you can visit this page: https://www.exitgames.com/Download, for downloading client and server SDKs:
Download the first link, the Photon Server SDK exe. This is a self-extracting archive, so after downloading simply browse to the location you want to extract the server code to.
You'll also want to download the Unity3D SDK, so find the download link among the list of client SDKs and download it.
After downloading the client SDK, navigate to libs | Release and drop the
Photon3Unity3D.dll file into a Plugins folder in your Unity project.
You will also want to download the server license file, so choose the first on the download page (100 CCU, ...