Hello Anuj,
Please use bleow code :-
CALL FUNCTION 'CONVERSION_EXIT_ALPHA_INPUT'
EXPORTING
INPUT = gs_tab1-vbeln
IMPORTING
OUTPUT = gs_tab1-vbeln,
CALL FUNCTION 'BAPI_CUSTOMERCONTRACT_CHANGE'
EXPORTING
SALESDOCUMENT = gs_tab1-vbeln
CONTRACT_HEADER_IN = gs_header
CONTRACT_HEADER_INX = gs_headerx
TABLES
RETURN = gt_return
CONTRACT_ITEM_IN = gt_item
CONTRACT_ITEM_INX = gt_itemx.
Thanks