Set the Direction property on the TextBoxAnnotation object:

DocumentInfo doc = Document.GetDocumentInfo(entryId, session);
PageInfo page = doc.GetPageInfo(1);

foreach (AnnotationBase ann in page.GetAnnotations())
{
    if (ann.AnnotationType == AnnotationType.TextBox)
    {
        TextBoxAnnotation textbox = (TextBoxAnnotation)ann;
        textbox.Direction = TextDirection.BottomToTop;
        textbox.Save();
    }
}
page.Save();