Texts are represented by TextBox objects. You may change TextBox.Style.Font property or override font using TextBox.FormattedText propery value. If text belongs to Border, Title block or sketched symbol you have to edit corresponding definition: BorderDefinition, SketchedSymbolDefinition or TitleBlockDefinition objects. There are several VBA samples in API help (e.g., CreateSketchedSymbolDefinition, CreateTitleBlockDefinition).
Open drawing and run the following sample. It will dump information about texts in current title block and the change font type and size for Author field.
Sub OverrideTextFont ()
Dim oDoc As DrawingDocument
Set oDoc = ThisApplication.ActiveDocument
Dim oSheet As Sheet
Set oSheet = oDoc.ActiveSheet
Dim oTB As TitleBlock
Set oTB = oSheet.TitleBlock
Dim oTBDef As TitleBlockDefinition
Set oTBDef = oTB.Definition
Dim oSketch As DrawingSketch
Set oSketch = oTBDef.Sketch
'dump texts in this title block
Dim oTextBox As TextBox
Dim i As Integer
For i = 1 To oSketch.TextBoxes.Count
Set oTextBox = oSketch.TextBoxes.Item(i)
Debug.Print i, oTextBox.Text
Debug.Print i, oTextBox.FormattedText
Debug.Print oTextBox.Style.Name
Debug.Print
Next
' change formatted text for box #9 ('Author' in my case)
' override font family and size
Call oTBDef.Edit(oSketch) ' enter edit mode
Set oTextBox = oSketch.TextBoxes.Item(9)
' text value is derived from to iPropertyc "Author"
oTextBox.FormattedText = "<StyleOverride Font='Chiller' FontSize='1'><Property Document='drawing' PropertySet='Inventor Summary Information' Property='Author' FormatID='{F29F85E0-4FF9-1068-AB91-08002B27B3D9}' PropertyID='4'>AUTHOR</Property></StyleOverride>"
Call oTBDef.ExitEdit(True) ' exit edit mode
Beep
End Sub
Hope this could help you to develop your own procedure.
Cheers,
Vladimir Ananyev
Developer Technical Services
Autodesk Developer Network