The correct answer is remove the dust inside the PC. It is of extreme importance to remove the dust particles inside the pc, if you want your pc to avoid getting over heated. This is because at times, the dust particles can themselves become heated due to the internal heat of the computer.
Being better thermal conductors, the dust particles can go to a pretty high temperature which again transfers the heat around to the pc components hence damaging it in the long run. There are many ways to remove the dust. Most common method of dust removal is the usage of compressed air.