Start by turning off the power to the furnace and allowing it to cool down completely. Use a vacuum cleaner with a brush attachment to remove any dust, dirt, cobwebs, or debris that may have accumulated on the unit. If stubborn deposits or soot buildup are on the heat exchanger, you can use a suitable mild cleaner or a cleaning solution specifically designed for heat exchangers. Rinse the heat exchanger with water, ensuring that the water flows from the top to the bottom to wash away any remaining residue or cleaning solution. Use a hose to create a seal at the bottom of the heat exchanger and let the water pour out from the bottom. Once the cleaning process is complete, thoroughly dry the heat exchanger before reassembling and restarting the furnace. It's worth noting that cleaning a furnace heat exchanger can be a complex task, and it's recommended to seek professional assistance to ensure proper cleaning and safety.