I'm not familiar with that specific incubator. I think that is one where you raise the humidity by increasing the water surface area. With mine I have four different water reservoirs in the bottom, I control humidity by how many of those reservoirs have water in them. They are different sizes so which reservoirs I use controls the humidity.
It doesn't matter what the depth of the water is in those reservoirs, just how much surface is there for water to evaporate from. I don't know what your owner's manual says about that. If you don't have an extra reservoir you can fill with water you might be able to put a sponge, paper towel, or small washcloth with part in that reservoir to wick water out where it can evaporate. Or can you put a small container in there to create a new reservoir.