Update multiple rows using select statement in Oracle

Im using this script for updating "gndt_customer_identification" table, value of V_iden_no coming from select statement. How i can update multiple row when select return more then one value for V_iden_no ???

Update gndt_customer_identification
set V_iden_code = 'CNIC', 
    V_iden_no = (
        SELECT SUBSTR (V_iden_no,1,13)
        FROM gndt_customer_identification
        WHERE V_iden_code = 'DM_OT'
        AND V_iden_no LIKE '_____________ _%' 
        and N_CUST_REF_NO =10434997
        )
where N_CUST_REF_NO =10434997;