How do I reduce voltage from 5V to 1V using a resistor?
The humidity sensor in the question operates on an AC signal of up to 1 Volt RMS. The datasheet specifically mentions an operating frequency range of 0.5 to 2 KHz, as well.

If the sensor is operated with DC supply, one electrode (the negative one, if I remember correctly) will deteriorate rapidly due to ion migration towards one plate in preference to the other, rendering the part inoperative.

Now, regarding a suitable operating mechanism: 

The impedance curve of the device spans a range of anywhere from 1 to 10 Megaohm at 20% relative humidity, down to between 1 and 5 kΩ at 90% RH. The impedance table in the datasheet specifies values from 1.1 kΩ to 7.2 mΩ, too broad a span for a voltage divider to work. 

Calculating current through the device for 1 Volt across it, spanning this impedance range to worst-case limits:
At 90% RH, for 1 kΩ, I = 100 μA
At 20% RH, for 10 MΩ, I = 100 nA

Thus, a very low impedance (100 Ohm or less) AC voltage source would be needed, to drive this sensor suitably, if it is to be operated in voltage driven mode. This shows that a voltage divider would be a very inefficient, and somewhat ineffective, way of driving the sensor.

Instead, a more viable approach would be to drive the sensor using a current source, with the DC blocked using a suitably large capacitor. 

There are several current source circuits out there, using bijunction transistors, FETs, or op-amps. Pick one that suits your purpose and budget, gate the current with an input from one pin of your microcontroller being toggled at say 1 KHz, and read the voltage across the sensor using an ADC pin of the MCU. 

Note that this will not give very precise results, as such electrode-based humidity sensors are characterized using a bipolar sine wave. Improvements to the solution could include using an RC or LC filter to bypass the higher harmonics of the 1 KHz signal, leaving an approximation of a 1 KHz sine wave. 

Actually designing such an AC (near)sine wave, stiff current source is left as an exercise to others less preoccupied than me.