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.

Do not do the latter operation without blocking the wings otherwise the fan could be damaged by the turning speed produced by the high airflow generated by an air compressor. Moreover, never try to clean the fan with an air compressor when the laptop is not dismounted, because that speed could generate electricity that could damage the motherboard!

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.