I am only posting this as a demonstration that this can be done. I am by no means stating that this will work for anything other than my small test I have done with the code.

- It will ask for the source file path
- It will ask for a full path for the output file it's going to create
- It will ask the string you want to find. I used "test"
- It will ask the string you want to replace it with. I used "TEST"
- It will ask you if there's a delimiter between data in the rows
- It will process the data until it's done. (I was too lazy to add any error checking which is the reason for the disclaimer below)

VERY BIG DISCLAIMER: USE AT YOUR OWN RISK
If you want to test this, I suggest copying the file and run the test on a duplicate so the original remains unaffected.

Macro:
Dim FilePath As String
    Dim FilePathTo As String
    
    Dim findthis As String
    Dim replwiththis As String
    
    Dim delimiter As String
    
    FilePath = InputBox("What is the full path of your .txt file?")
    If Dir(FilePath) = "" Then
        MsgBox "File does not exist, exiting"
        End
    End If
    FilePath2 = InputBox("What do you want to call the output file? (new file)")
    If Dir(FilePath2) <> "" Then
        MsgBox "File Already Exists, exiting"
        End
    End If
    
    findthis = InputBox("What value do you want to find?")
    replwiththis = InputBox("What value do you want to replace it with?")
    delimiter = InputBox("What is the delimiter? (Enter nothing if none)")
    
    Open FilePath2 For Output As #1
        Open FilePath For Input As #2
            Do Until EOF(2)
                Line Input #2, LineFromFile
                Dim newline As String
                
                LineItems = Split(LineFromFile, delimiter)
                If UBound(LineItems) >= 0 Then
                    For i = LBound(LineItems) To UBound(LineItems)
                        If LineItems(i) = findthis Then
                            LineItems(i) = replwiththis
                        End If
                    Next i
                    
                    newline = Join(LineItems, delimiter)
                    Print #1, newline
                
                Else
                    If Len(LineFromFile) < 1 Then
                        Print #1, ""
                    Else
                        Print #1, LineFromFile
                    End If
                End If
                
            Loop
        Close #2
    Close #1
    MsgBox ("Done")
Input File:
test,test1,test2,test3
test4,test5,test4,test3
test2,test1,test,test
Output File:
TEST,test1,test2,test3
test4,test5,test4,test3
test2,test1,TEST,TEST