string.IndexOf(SearchFor as string) will give you the index of first occurrence of the argument in the string, then you can use substring to manipulate it.
Log(ReplaceFirst("3434abc abd", "434", "dddd"))
Sub ReplaceFirst(s As String, SearchFor As String, ReplaceWith As String) As String
Dim i As Int = s.IndexOf(SearchFor)
If i > -1 Then
Return s.SubString2(0, i) & ReplaceWith & s.SubString(i + SearchFor.Length)
Else
Return s
End If
End Sub