```javascript
let link = document.querySelector("link[rel~='icon']")
if( link ) link.href = "new_icon.png"
else { // see keparo's answer at: https://stackoverflow.com/questions/260857/changing-website-favicon-dynamically
  link = document.createElement("link")
  link.setAttribute("rel") = "icon"
  link.href = "icon.png"
  document.head.append(link)
}
```