Vb 如何只替换特定一行的某个字符？

稍微改了下
Private Sub Command4_Click()
Dim str As String
Dim start_str As String '想替换的
Dim to_str As String '想替换成的
Dim index() As String
Dim i As Integer, j As Integer
i = 1
j = 0
ReDim index(0 To 100)
str = "abcdeft123sdfhi123dshfi123"
start_str = "ft"
to_str = "FF"
Do While i > 0
i = InStr(i + Len(start_str), str, start_str)
If i > 0 Then
index(j) = i
j = j + 1
End If
Loop
For i = 0 To UBound(index)
If index(i) = "" Then
ReDim Preserve index(0 To i)
Exit For
End If
Next i
i = Val(InputBox("找到" & UBound(index))) & " 个匹配的字符,想替换第几个?"
If i > 0 And i <= UBound(index) Then
str = Left(str, CInt(index(i - 1)) - 1) & Replace(str, start_str, to_str, CInt(index(i - 1)), 1)
MsgBox "替换后的字符串为:" & str
End If
End Sub