In the Computers & Contacts window > Click the drop-down arrow next to your username > Select Sign Out
New Interface (preview)
Click your profile picture on the top left (doesn't look like a button but it is) > Click the Gear Icon on the top right > Click Sign Out
It took me ten minutes to find it. I did so just before this search result came up. Aren't new versions supposed to be easier to use than the previous versions?