Hello Jayant,
I think you should be using BAPI_SALESORDER_CREATEFROMDAT2.
Now coming to your point on data not getting updated.
You will have to pass UPDATEFLAG = 'U' or 'I' in the parameter ORDER_ITEMS_INX in your input parameters.
You ca have a look at the documentation on BAPI_SALESORDER_CREATEFROMDAT2 and you will have a better idea how to use it and what are the constraints.
Also are you doing BAPI_TRANSACTION_COMMIT after you save the order?
Thanks
Amit