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FOR every .txt-file in the folder
   REPLACE every string "aaa" in the file with "bbb" and
   WRITE the new version of the file into the folder X:\somewhere\
   COMMENT: Original file shall not be changed
END LOOP

Loop, FolderPath\*.txt, 0, 1   ;<--The 0 means it only looks at files, change 1 to 0 if you don't want to look in sub folders
{
    tempContents := ""       ;Clear the temp variable for each new file
   Loop, Read, %A_LoopFileLongPath%     ;Read each line in the file
      tempContents .=        ;Append each line to the variable
   StringReplace, tempContents, tempContents, aaa, bbb, All   ;Replace each aaa with bbb in this variable
   FileAppend, %tempContents%, Destination Path\%A_LoopFileName%    ;Write the data to a new file
}

tempContents .=       

   Loop, Read, %A_LoopFileLongPath%     ;Read each line in the file
      {
   tempContents .= A_LoopReadLine       ;Append each line to the variable
      tempContents .= "`r`n" ; New line after each line, otherwise all text wil be in one single line
   }