Hello Praveen Arun,
For modifying the SC Smartform check with BBP_SC if you are using the standard smartform. Basically most of them uses the standard one. So I don't think there would be any change. Now for modifying it you would require an access key or you can copy it any create your a Zsmartform and later on you can call your Zsmartform through the button Print Preview.And for doing so you can implement a BADI BBP_CHANGE_SF_SC and then you can code --> for example
Method IF_EX_BBP_CHANGE_SF_SC~CHANGE_SMARTFORM.
if smartform = 'BBP_PO'.
smartform = 'ZBBP_PO'. " <<-- change the name of your custom smart form here
endif.
if smartform = 'BBP_SC'.
smartform = 'ZBBP_SC'. " <<-- change the name of your custom smart form here
endif.
ENDMETHOD.
I hope this will solve your issue 
Thanks
Gaurav Gautam