My experience is as follows:
# Disconnect the fan (they spin and generate power but I always err on the side of caution)
# remove the fan(s).
# pull any hard grab-able debris with tweezers.
# then use a can of pressurized air to blow or a vaccum to pull the dusty bits. Just remember the compressed air will blow the dust all over.
# reverse the process…. but before I do this, I usually check heatsink (if easily removed) to insure the heatsink compound isn’t hard. DO NOT DO THIS if you don’t have replacement compund handy.