I have two sleeping units standing behind another unit each on the same field. I try to click on their shields but Civ 5 always focuses on the unit in front of it. How can I select a unit behind another unit? (In both cases it's a civilian unit behind a military unit.)
Usually when you have two units stacked on a tile, you can pick the one behind it by clicking on the icons. Civilian units generally have triangular icons, while military units have circle or shield-shaped (if they're fortified) icons. You can also generally click the tile repeatedly to cycle through the units on that tile, if you so desire.
In certain cases, such as when a unit is activated during the "complete automated tasks" portion of a turn, you won't be able to select just any unit - usually, trying to select another unit will cause the selection to jump back to the previously selected unit. You'll end up in this situation if you gave a long move order to a unit and they can no longer complete it, for example.
First method: You need to click above the shield (not on it). Move the cursor so that it's a few pixels above the shield in the back. You'll see the behind shield get the pulsing glow. It may also throw up a tool tip that tells you to click to select that unit.
Second method: Go into the Military Overview (F3) and click on the unit. When you close the overview window, that unit will be selected. (Actually, it gets selected while you're still in the overview. The main screen gets dimmed behind that window but you can still see the unit being selected in the bottom left corner.)