foreach (Field mergeField in document.Fields)
    {
       if (mergeField.Type == WdFieldType.wdFieldMergeField)
       {
          string fieldText = mergeField.Code.Text;
          string fieldName = Extensions.GetFieldName(fieldText);

          if (values.ContainsKey(fieldName))
          {
             mergeField.Select();
             application.Selection.TypeText(values[fieldName]);
          }
       }
    }

subscriptionDocument.Sections[1].Headers[WdHeaderFooterIndex.wdHeaderFooterPrimary].Range.Fields.Count;

?activedocument.Sections(1).Headers(wdHeaderFooterFirstPage).Range.Fields.Count

putField("First_Name", "Fred");
putField("Last_Name", "Bloggs");

private void putField(string search, string replace) {
    foreach (Section section in doc.Sections) {
        doReplace(section.Range.Find, search, replace);
    foreach (HeaderFooter h in section.Headers) {
        doReplace(h.Range.Find, search, replace);
    }
     foreach (HeaderFooter f in section.Footers) {
        doReplace(f.Range.Find, search, replace);
    }
    }
}

private void doReplace(Find fnd, string search, string replace){
        fnd.ClearFormatting();
        fnd.Replacement.ClearFormatting();
        fnd.Forward = true;
        fnd.Wrap = WdFindWrap.wdFindContinue;
        fnd.Text = "«" + search + "»";
        fnd.Replacement.Text = replace;
        fnd.Execute(Replace: WdReplace.wdReplaceAll);
}