Changing H3 Headings to H2 Headings with Google Apps Script

function changeH3ToH2() {
  // Get the active document
  var doc = DocumentApp.getActiveDocument();
  var body = doc.getBody();
  
  // Get all the paragraphs in the document
  var paragraphs = body.getParagraphs();
  
  for (var i = 0; i < paragraphs.length; i++) {
    var paragraph = paragraphs[i];
    
    // Check if the paragraph is an H3
    if (paragraph.getHeading() === DocumentApp.ParagraphHeading.HEADING3) {
      // Change the heading to H2
      paragraph.setHeading(DocumentApp.ParagraphHeading.HEADING2);
    }
  }
}