The laws of thermostat controls are the same as those of an electric timer. Some poskim maintain that one may not adjust a thermostat. According to R. Shlomo Zalman Auerbach and others, one may change the setting if it will lead to the extension of the current state.
For example, if one set the thermostat of an electric oil-filled radiator to medium before Shabbat, but on Shabbat realizes that it is hotter than he had expected, he may lower the setting of the thermostat once it has switched the radiator off. Thus, he ensures that the radiator will remain off for a longer period of time, and the heating element will work for a shorter period of time. However, one may not lower the thermostat while the radiator is on, because this changes the current state by causing the heating element to cycle off sooner.
If, during the course of Shabbat, one wants a radiator to stay on for longer, he must wait until the radiator has cycled on, and the temperature of the radiator has reached at least yad soledet bo (at least 71 degrees Celsius). Then he may turn up the thermostat so that the radiator will stay on for longer. However, if he does this when the temperature is below yad soledet bo, he transgresses Bishul, because he is causing the oil inside the radiator to heat up. Once the radiator has cycled off, it is not permissible under any circumstances to turn up the thermostat, because this will change the current state and may lead to the radiator turning on immediately.
The same rules apply to air conditioners and refrigerators that have manual thermostats. When the compressor has cycled on, one may turn down the temperature, which will keep the refrigerator or air conditioner on longer. When the compressor has cycled off, one may turn up the temperature, which will keep the machine off longer (Minĥat Shlomo §10; SSK 23:24).
Of course, all of this assumes that there is no display recording the temperature. However, if the thermostat is adjusted by pressing buttons to change the temperature, and this is shown in an electronic display (as is the case with many air conditioners), then it is prohibited, both on account of Kotev, and because each press of a button makes direct use of electricity.