@echo off
set row=0
set month=%date:~5,2%
set day=%date:~8,2%
type nul>"C:\test\b.txt"
setlocal enabledelayedexpansion
for /f "usebackq tokens=1,2* delims=:" %%a in ("C:\test\a.txt") do (
  set str=%%a
  set /a row+=1
  if !row! == 50 (
    set str=!str:~0,5!%month%!str:~7!
  ) else if !row! == 52 (
    set str=!str:~0,5!%day%!str:~7!
  )
  >>"C:\test\b.txt" echo !str!
)
move /y "C:\test\b.txt" "C:\test\a.txt"