Incorrect floors count

I work on the 3rd floor and usually go by stairs to my work place. But the floor count in GC is far away from the reality. It never counts floors in the morning when I go to my job (always zero). In the midday, when I go for a lunch in sometimes counts correctly, but more often not. Is there any way to fix this? For instance to calibrate altimeter or something else?

The device is FR945

The SW version is 4.00