From the instruction manual:
The GPS green location information in the
product is determined on the actual course,
when the information is collected. In case of
changes in the green location, you can use
this function to relocate it.
For Example: If you are going to correct
the distance of the green centre of the third
hole on a course, press the UPWARD and
DOWNWARD arrow buttons to choose the
third hole on the start menu after you get
to the green centre, then press and hold
PAGE for three seconds. The figures will
start flashing, which indicates that the green
correction function has started.
Press the UPWARD and DOWNWARD arrow
buttons to change the flashing figures into the
green centre. Press the ENTER button again.
The device will ask you to confirm whether
you want to correct the green centre. When
you select <OK>, the distance will be reset as
<0>, which means the distance between you
and the green centre is 0.