sn. @berduş çözümü güzel çalışıyor

Private Sub degis()

    Dim intSira As Integer, txtMtn As String, txtBul As String, txtDgs As String

    

    txtMtn = "MUVAFFAKİYETSİZLEŞTİRİCİLEŞTİRİVEREMEYEBİLECEKLERİMİZDENMİŞSİNİZCESİNESİNİZ"

    intSira = 8

    txtBul = "İ"

    txtDgs = " $€£ "

    

    MsgBox degisHrf(txtMtn, intSira, txtBul, txtDgs)

    

End Sub



Function degisHrf(txtMtn As String, intSira As Integer, txtBul As String, txtDgs As String) As String

On Error GoTo Err_hata

    Dim result As String



    Dim x, y As Integer



    y = 0

    

    For x = 1 To intSira

        y = InStr(y + 1, txtMtn, txtBul)

    Next x

    

    result = Left(txtMtn, y - 1) & txtDgs & Mid(txtMtn, y + Len(txtBul))



Exit_kod:

    degisHrf = result

    Exit Function



Err_hata:

    result = txtMtn

    MsgBox Err.Description

    Resume Exit_kod

End Function