The following will set the X coordinate of the reference point to 0:
 daylgtctrl = idf_file.idfobjects["Daylighting:ReferencePoint".upper()][0]
daylgtctrl.XCoordinate_of_Reference_Point = 0
Daylighting:ReferencePoint,
Core_Retail_DaylRefPt1,    !- Name
Core_Retail,              !- Zone Name
0,                        !- XCoordinate of Reference Point
14.2,                     !- YCoordinate of Reference Point
0.0;                      !- ZCoordinate of Reference Point
 You can also get all field names of an object are by using .fieldnames:
 daylgtctrl.fieldnames
['key',
 'Name',
 'Zone_Name',
 'XCoordinate_of_Reference_Point',
 'YCoordinate_of_Reference_Point',
 'ZCoordinate_of_Reference_Point']
 If you know the index of the field, you can use it instead of its name as such:
 daylgtctrl[daylgtctrl.fieldnames[3]] = 0
Daylighting:ReferencePoint,
Core_Retail_DaylRefPt1,    !- Name
Core_Retail,              !- Zone Name
0,                        !- XCoordinate of Reference Point
14.2,                     !- YCoordinate of Reference Point
0.0;                      !- ZCoordinate of Reference Point