I clean my S9 miners every 8 weeks.

Turn off and unplug, allow to cool.
Remove fan near power supply connectors, take the adapter plate off the miner body X 4 screws.
Remove cables connecting hash cards to control card X 3.
Slide out hash cards.

Carefully brush hash cards using anti static brush, remove all dust and debris.
Blow with canned air.

Remove other fan and clean with brush, wipe blades with suitable cleaner.
Clean inside of case if necessary.

Put back together in reverse order, cross fingers and run up.

Pretty easy to do after a couple of tries.