Put the favicons in an /image directory inside the /public directory and put the code below in your _app.js
<Head>
<link rel="shortcut icon" href="/images/favicon.ico" />
<link rel="apple-touch-icon" sizes="180x180" href="/images/apple-touch-icon.png" />
<link rel="icon" type="image/png" sizes="32x32" href="/images/favicon-32x32.png"/>
<link rel="icon" type="image/png" sizes="16x16" href="/images/favicon-16x16.png"/>
</Head>