Hi Janaraja,
Check these Information Notes also Sample Code given by SAP :
Note 1083986 - BAPIs for classification - Overview
Note 1323945 - BAPI_CLASS_CREATE: Instructions for use
Note 1479120 - BAPI_CHARACT_CREATE: Operating instructions
Note 1235844 - BAPI_CLASS_CHANGE: Operating instructions
As per your requirement , if you already create class using "BAPI_CLASS_CREATE" without Characteristics .
then you create two characteristics using "BAPI_CHARACT_CREATE" .
Now you want to assign these characteristics to class , mean you must change your class with assign these characteristics, for Changing the Class use
BAPI_CLASS_CHANGE. if you use this Class change BAPI , use one more BAPI (i.e. "BAPI_CLASS_GETDETAIL") before assign any value into "BAPI_CLASS_CHANGE" .
Check the Sample Code in Note 1235844 - BAPI_CLASS_CHANGE: Operating instructions
Regard's
Smruti