To exit a building at any time (no matter how far into it you are) click the Exit button below the mini-map.
The exit button looks like a globe with a green arrow on it and is to the right of the rest “heart shaped” icon, the exit button now looks like this:
After clicking the button you will find yourself outside.
Note: Those of you used to having the Exit button at the bottom of the screen will simply have to get used to the new way, which should eliminate accidentaly clicking.