Block the wings of the fan ─ I have used a piece of iron wire but a cotton swab or a pencil would work too ─ and dust it off using an air compressor or a vacuum.

Once you have dust the fan off, unblock it and clean each wing using cotton swabs dipped in alcohol 70% in order to remove the residual dirt. If it is afterwards necessary, block the wings and dust the fan off again using an air compressor to remove eventual cotton-fibers leftovers.