maybe this helps you getting started
{code}
Dim a, b, n
For Each b In ThisDrawing.ModelSpace
If TypeOf b Is AcadBlockReference Then
a = b.GetAttributes
For n = 0 To UBound(a)
If a(n).TagString = "TAG1" Then
If a(n).TextString = "VALUE" Then
a(n).TextString = "NEW_VALUE"
End If
End If
Next n
End If
Next b
{code}