The best way would be to take it apart and clean the fan and the heatsink with a dry brush. If you blow it with a compressor the fan could get stuck from the dust clogs so it's a very bad idea. Before puting back the heatsink be sure to clean the CPU very well from all the old paste, and the heatsink as well. After that apply some fresh paste (not much) then put it all back together. Make sure all the screws are screwed well. After turning it on, check the temps with HWMonitor.