Here is the procedure we performed over the phone to fine tune the ICOM-R20 receiver :

Change the tuning resolution
1) Press and hold the "9" button to access the "TS" menu. 

2) Set the tuning step to 1.0 kHz by turning the R Dial on the top of the receiver.

3) Press the "9" button again to save the change.

Switch to VFO mode
4) Press the "VFO MHZ" button to switch the receiver to VFO mode. This causes the receiver to act like a normal radio dial whose frequency can be changed manually with a knob.

5) The receiver should read "MODE FM." If it is in a different mode, press the "MODE SCAN" button until it reads "MODE FM."

6) Use the button pad to program your desired radiosonde frequency. For example, if the radiosonde is set to 402 MHz, press the numbers so that the screen reads "402.000"

Fine Tune the Receiver
7) Turn on the radiosonde.

8) Start the software and select the radiosonde type.

9) Using the R Dial on top of the receiver, fine tune the receiver until the packet reception is above 90% on the ground. Typically, the value will be within 3 kHz one way or the other or no change will be necessary. For example, a radiosonde set to 402.000MHz will usually be between 401.997MHz and 402.003MHz.