Oh. Crap. U need to know how to do this.
There are 2 ways:
The Slavery Civic (enabled by Bronze Working), also referred to as "whipping". Once you adopt this civic (through the Civic Screen OR when you're done researching BW), you can enter the City Screen and if the Slavery button is lit, you can remove some population for huge production, automatically bringing whatever ur building to 1 turn till completion. This results in 1 unhappy face for 10 turns on normal speed. It can be used to remove unhappiness or unhealthiness by killing the population.
The second way is by using the Universal Suffrage Civic. This allows you to pay for this extra production with gold, like in AC.
Hope this helps!