How to pause Steam downloads?
Solution 1:
Instead of viewing the games through your "Installed" or "All" games dropdown, select "Downloads" where you can manage all downloads Steam is performing. Here you can pause and resume or reorder any active or pending downloads.
Additionally, launching a Steam game will, by default, pause all current downloads.
Solution 2:
Go to steam "Big Picture Mode". Click anything that is being downloaded (the one who's not supposed to be downloading). Click "Downloading". You'll see the pause button in there!