Augmenting a capacitance base humidity sensing circuit
RH capacitance sensors increase in capacitance with humidity. 

To make it read lower you could add a capacitor in series. The total capacitance is, of course, \$C_S = \frac {C_1 C_2}{C_1 + C_2}\$. 

So, very roughly to decrease the capacitance by 10%, add a capacitor in series that is about 10x the value. So if it's 20pF, add 200pF in series to get about 18pF. 

But if the sensor is broken that is not going to help you much.