In Civ4, I can easily reassign citizens to benefit from a different tile. For example, from a forest tile to a mountain tile to increase production with the cost of decreasing food. I have not been able to find a way to do so in Civ5. Everything so far has been assigned automatically by the game. I've tried clicking everywhere with either my left or right mouse button in the city screen. Is there a secret button I am not aware of?
On the top right of the city screen is an "Citizen Management" button:
After you click it, it shows you which tiles has a citizen on them, and allows you to: