Steam: The process on Steam is relatively simple: go to your Steam games folder (the default is C:\Program Files\Steam\SteamApps, though I always install Steam to C:\Games instead).

Navigate to Settings > General. Under Game install options, you can change your default installation folder. Note that any games already installed will automatically be moved to this folder as well.

Want to bulk or batch install multiple games at once in Steam on PC? See this post for the step-by-step instructions you need to follow.