Is it possible to manually adjust the fan speed (fan RPM) in order to reduce the temperature of the miner? / Is there any available firmware that can increase or decrease the fan speed (fan RPM)?

It is not possible to manually adjust the fan speed (fan RPM) of the miner. In fact, the fan speed (fan RPM) is automatically adjusted according to the miner's temperature. 

Additionally, there is no firmware available that can increase or decrease the fan speed (fan RPM).

You can check the miners' normal operating temperature here. Miner's normal operating temperature

As long as the temperature remains within the normal range, there is no need to worry about overheating. Our miners are equipped with high-temperature protection mechanisms and will automatically shut down if the temperature exceeds the safe threshold.

If you're concerned about burnout issues, please refer to:

What Could Cause a Miner’s Hashboard to Burn Out?

What causes the miner's 6-PIN power connector to burn out?