Fan runs constantly: is there a guide for cleaning out dust?

I have been advised that the fan running constantly indicates the need to clean out dust from inside the case. Does there exist a guide for doing this?

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I would install this great app so you can see why your systems fan is running hard TG Pro Take a snapshot of the apps main window making sure to capture all of the sensors and post it here Adding images to an existing question. Then we can figure out what is needed.