Oracle 23c

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CREATE TABLE txn_method (txn_mthd_code) AS
  SELECT 'TXN_MTHD_' || LEVEL FROM DUAL CONNECT BY LEVEL <= 35;
35 rows affected

UPDATE TXN_METHOD
SET    TXN_MTHD_CODE = 'TXN_MTHD_'
                     || LPAD(SUBSTR(TXN_MTHD_CODE, LENGTH('TXN_MTHD_') + 1), 3, '0')
WHERE  TXN_MTHD_CODE LIKE 'TXN_MTHD_%';
35 rows affected

SELECT *
FROM   txn_method;

TXN_MTHD_CODE

TXN_MTHD_001

TXN_MTHD_002

TXN_MTHD_003

TXN_MTHD_004

TXN_MTHD_005

TXN_MTHD_006

TXN_MTHD_007

TXN_MTHD_008

TXN_MTHD_009

TXN_MTHD_010

TXN_MTHD_011

TXN_MTHD_012

TXN_MTHD_013

TXN_MTHD_014

TXN_MTHD_015

TXN_MTHD_016

TXN_MTHD_017

TXN_MTHD_018

TXN_MTHD_019

TXN_MTHD_020

TXN_MTHD_021

TXN_MTHD_022

TXN_MTHD_023

TXN_MTHD_024

TXN_MTHD_025

TXN_MTHD_026

TXN_MTHD_027

TXN_MTHD_028

TXN_MTHD_029

TXN_MTHD_030

TXN_MTHD_031

TXN_MTHD_032

TXN_MTHD_033

TXN_MTHD_034

TXN_MTHD_035