use the lux slider in the [F1] screen.
Increase lux spending to keep your core cities working, the corrupt outlaying cities are better served with specialist.
(it need not always be a clown, you can also use a scientist or tax-man, the specialist themself are always counted as content)
This will only work well if you bothered to build enough roads. Make sure every citizen works a roaded tile, so build enough workers to improve your land quickly.
A city will be in disorder the next turn if the number of unhappy people is greater than the number of happy people. (content people don't matter)
Both the lux slider and the specialist are mostly temporary solutions on the long term, you should be looking for lux resources, collect at least 1 of all 8 different types, either though trade or conquest. each different type of lux resource will provide 1 happiness effect to each city that is connected to your trade network, either via roads, harbors, or airports.
A marketplace will increase this effect even more.
Under some forms of government, you can turn unhappy citizens content with the presence of military units in the city. Each unit will give 1 contentness effect to a maximum that is different for each government type.
If you really have nothing better to build, some wonders and city improvements will also provide contentness.
