This sounds like a typical electric heater with a heating element and a fan that circulates air over the heating element to get that heat out into the area to be heated.
If the fan recirculates air within an enclosed space, then slowing the fan down won't increase the temperature within that enclosed space, it'll just make the temperature less uniform.
On the other hand, if the fan is drawing in cool air from outside the proofer, blowing it over the heating coils, and then into the proofer, you may be able to adjust the temperature in the proofer by controlling the fan speed.
But you should be very careful with this. Electric heating elements in a heater of this sort are, indeed, cooled by the air being blown over them. If you reduce or stop the airflow, the elements heat up to a higher temperature, and that can destroy the elements or cause a fire. The heaters are designed to have this constant airflow to operate safely.
Is or was this system controlled by a thermostat of some sort?
Did it used to work, but has now stopped working?
You can simply block some of the airflow through the fan with something, but again, this, or slowing the fan could be dangerous and may not do you any good depending on how the system is set up.
I'd want to know more about this system before giving any advice about what to do to increase or control the proofer temperature.