BAPI_SALESORDER_CHANGE

I need to implement bapi_salesorder_change

if actioncode = 'aa' update delivery block to blank and customergroup2 to aa 
if actioncode = 'ac' update delivery block to blank and customergroup2 to ac ....

I dont know how to implement bapi change. I have been told I can either flag fields or compare fields. But I do not know how to write code to implement any of this. Could someone help me please.