Press the "FAN" button on your thermostat to engage the fan settings of the unit. For a central air conditioner, this unit will be on a wall somewhere in your home. For an in-wall unit or a window unit, this thermostat will typically be on the air conditioner itself.
Switch your air conditioner fan to "On." This means that the fan has been turned on and will remain on, circulating air even if the air conditioning unit itself is not producing any additional cool air to be circulated.
Switch your air conditioner fan to "Auto." This means that the fan will turn itself on and off as dictated by the thermostat. For example, when the air conditioner switches on and starts producing cool air, the fan will turn on as well and start circulating that air. When the desired temperature is reached, the air conditioner will stop and so will the fan.
Switch your air conditioner fan to "Off." As its name implies, this means that the fan has been turned completely off and will not turn back on until you manually switch it back to one of the other two aforementioned settings.
Look for any additional fan settings present on your thermostat. Most air conditioners only have the three aforementioned fan settings, but certain units will have additional settings depending on the model. These settings all related to the speed of the fan, which controls the rate at which air is circulated. For example, a "high" speed fan mode will blow air faster than a "medium" or "low" speed fan mode.