@echo off
set "File=a.txt"
set "Line=2"
set "Str=qwert"
set n=0

(for /f "delims=" %%a in ('findstr /n .* "%File%"') do (
  set /a n+=1
  set var=%%a
  setlocal enabledelayedexpansion
  if !n!==%Line% echo;!Str!
  echo;!var:*:=!
  endlocal
)) > "%temp%\~.txt"

move "%temp%\~.txt" "%File%"
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@echo off
setlocal enabledelayedexpansion
for /f "delims=" %%a in (a.txt) do (
         set /a temp_+=1
         if !temp_! == 2 echo qwert>>b.txt
         echo %%a>>b.txt
)
pause 
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@echo off&setlocal enabledelayedexpansion
for /f "delims=" %%a in (a.txt) do (
     echo %%a>>b.txt
     set n+=1
     if !n! equ 1 echo qwert>>b.txt
)
pause
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