Ideally, you'd start by defining an appropriate Paragraph Style or Character Style for the textbox content. You'd then apply that Style to all the textboxes. From then on, if you want to change any of the font attributes in the textboxes, all you'd need to do is to update the Style definition. No code required.
The programming problem at this stage, then, becomes one of applying your Style. For that you could use code like:
Code:
Sub Demo()
Dim Shp As Shape
For Each Shp In ActiveDocument.Shapes
With Shp
If .Type = msoTextBox Then
.TextFrame.TextRange.Style = "MyStyle"
End If
End With
Next
End Sub
where 'MyStyle' is your Style's name.