const replaceHeaderFooter = () => {
  // Returns the document with the specified ID
  const doc = DocumentApp.openById('DOCUMENT ID');

  // Retrieves the headers's container element which is DOCUMENT
  const parent = doc.getHeader().getParent();

  for (let i = 0; i < parent.getNumChildren(); i += 1) {
    // Retrieves the child element at the specified child index
    const child = parent.getChild(i);

    // Determine the exact type of a given child element
    const childType = child.getType();

    if (childType === DocumentApp.ElementType.HEADER_SECTION) {
      // Replaces all occurrences of a given text in regex pattern
      child.asHeaderSection().replaceText('{{Company}}', 'Digital Inspiration');
    } else if (childType === DocumentApp.ElementType.FOOTER_SECTION) {
      // Replaces all occurrences of a given text in regex pattern
      child.asFooterSection().replaceText('{{Copyright}}', '© Amit Agarwal');
    }
  }

  // Saves the current Document.
  // Causes pending updates to be flushed and applied.
  doc.saveAndClose();
};