Use this:
foreach (Word.Section section in document.Sections)
{
foreach (Word.HeaderFooter wordFooter in section.Headers)
{
foreach (Word.Paragraph para in wordFooter.Range.Paragraphs) // see the change of wordFooter in this line
{
Word.Range range = para.Range;
range.SetRange(1, 5);
range.Delete();
}
}
}