Actually I can answer my own question. From the manual:

You are able to adjust the outside temperature display plus or minus 5 degrees. Hold the select/reset knob for 10 seconds. The following sequences appear, 1 second at a time: 0,1,2,3,4,5,-5,-4,-3,-2,-1,0, 1 (0,1,2,3,-3,-2,-1,0,1)
When the temperature reaches the desired value, release the select/reset knob. You should see the new temperature displayed.