Sub Maybe()
Dim a As Long, b As String, bb As String, c As Range
a = Application.InputBox(Prompt:="Position (nth character) of character.", Title:="Enter a number.", Type:=1)
b = Application.InputBox(Prompt:="Enter the alphabetical letter to be replaced.", Title:="Letter.", Type:=2)
bb = Application.InputBox(Prompt:="Enter the alphabetical letter that replaces it.", Title:="Letter.", Type:=2)
For Each c In Range("A2:A" & Cells(Rows.Count, 1).End(xlUp).Row)
If Mid(c, a, 1) = b Then c.Value = Left(c, a - 1) & bb & Mid(c, a + 1, 99)
Next c
End Sub