ReplaceWord

Private Sub Command1_Click()
Dim oldstring As String, newletter As String, _
oldletter As String, newstring As String
oldstring = "hello To the world"
newletter = "YEAH"
oldletter = "hello"
newstring = Replace(oldstring, newletter, oldletter)
MsgBox newstring
End Sub

Public Function Replace(oldstring, newletter, oldletter) As String
Dim i As Integer
i = 1
Do While InStr(i, oldstring, oldletter, vbTextCompare) <> 0
Replace = Replace & Mid(oldstring, i, _
InStr(i, oldstring, oldletter, vbTextCompare) - i) & newletter
i = InStr(i, oldstring, oldletter, vbTextCompare) + Len(oldletter)
Loop
Replace = Replace & Right(oldstring, Len(oldstring) - i + 1)
End Function
Inputs:oldstring, newletter, oldletter