Cpu fan or power supply fan. Just open the side cover of the chassis.
General cpu fans can be replaced by themselves. Buy a similar size or one with radiator.
If it is a power fan, as long as you dare to remove it (it must be in the case of power failure), press the power button (discharge) several times to replace it. The point is that if the power plugs don't match, you need to cut off the original plug in the middle of the line, cut off the plug of the new fan, and then connect the old plug. Pay attention to the positive and negative poles and don't make a mistake.