Option Explicit
Const TARGET_FOLDER = "C:\Users\r\Desktop\a\a"
Const REPLACE_FROM = "\r\n"
Const REPLACE_TO = "s"

Const ForReading = 1 '読み込み
Const ForWriting = 2 '書きこみ(上書きﾓｰﾄﾞ)
Const ForAppending = 8 '書きこみ(追記ﾓｰﾄﾞ)

Dim objFSO, objFolder, objFile, objSubFolder, objTXT
Set objFSO = WScript.CreateObject("Scripting.FileSystemObject")
Set objFolder = objFSO.GetFolder(TARGET_FOLDER)

For Each objFile In objFolder.Files
 Dim strFilePath, infile, outfile, strData
 strFilePath = objFSO.BuildPath(TARGET_FOLDER, objFile.Name)
 Set infile = objFSO.OpenTextFile(strFilePath,ForReading)
 strData = infile.ReadAll
 infile.Close
 Set infile = Nothing
 Set outfile = objFSO.OpenTextFile(strFilePath,ForWriting) '(上書き)
 outfile.Write Replace(strData,REPLACE_FROM,REPLACE_TO)
 outfile.Close
 Set outfile = Nothing
Next

REPLACE_FROM の部分を､

Const TARGET_FOLDER = "C:\Users\r\Desktop\a\a"
Const REPLACE_FROM = "\r\n"  ' ★ここを
Const REPLACE_TO = "s"

このように変えてください｡

Const TARGET_FOLDER = "C:\Users\r\Desktop\a\a"
Dim REPLACE_FROM  ' ★こうする
REPLACE_FROM = Chr(13) + Chr(10)  ' ★こうする
Const REPLACE_TO = "s"