loop through cursor

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Oracle

CREATE OR REPLACE TRIGGER DOTCOM.TRG_CHK
BEFORE INSERT OR UPDATE
ON DOTCOM.SM_SRET_DETAIL REFERENCING NEW AS NEW OLD AS OLD
FOR EACH ROW
DECLARE
CURSOR C2
   IS
   SELECT SD_SRET_QTY_BU,SD_QTY_BU FROM SM_SALES_DETAIL
   WHERE SD_SH_SYS_ID = 7777--SDSYSID
   AND SD_ITEM_CODE   = 'DOTFULL0064'--SDITEMCODE
   AND SD_LOC_CODE    = 'D01'--SDLOCNCODE
   for update of SD_SRET_QTY_BU;

BEGIN

     for rec in c2 loop
                VSD_QTY_BU := rec.SD_QTY_BU;
            VSD_SRET_QTY_BU := rec.SD_SRET_QTY_BU;

                IF   (VSD_QTY_BU - VSD_SRET_QTY_BU) >= 0 THEN
                    UPDATE SM_SALES_DETAIL SET SD_SRET_QTY_BU = SD_SRET_QTY_BU + newval
                WHERE current of c2;

             ELSE
                    RAISE_APPLICATION_ERROR (-20002,' RETURN QUANTITY EXCEEDED SOLD QUANTITY  ' || :NEW.CSRI_ITEM_CODE);



                END IF;
            END LOOP;
 END

When I run this the cursor will return 2 rows, but I want to update both of this row seperately based on a condition QTY_BU and RET_QTY_BU..only this two columns are different for the two rows.
When I try to update it is updating on both the row.