What make and model incubator do you have? That way, people familiar with that incubator can give you pretty specific advice.
I don't know what you mean by it being fully automatic. It sounds liek it is not like my Hovabator 1588 which has reservoirs in the bottom. I control the humidity by which size reservoir I fill. In this type, water surface area is what controls humidity. To increase humidity you can fill more reservoirs or add a sponge or towel that will wick the water out and provide more surface area so more water can evaporate. To drop humidity, take that stuff out or just use smaller reservoirs. It may take a while for a larger one to go dry. Another way to immediately drop the humidity without waiting for it to totally dry out is to cover it, maybe with foil to reduce the surface area.
If you have another type of incubatorm this won't help you at all.
Good luck!