Change the meta description on a specific page?

The meta description isn’t part of the Twenty Fifteen theme.

You mentioned the Yoast plugin in a previous question yesterday, so I guess the meta description comes from there.

According to their API page, there’s a filter called wpseo_metadesc that might be what you’re looking for.

Here’s an untested example:

/**
 * Change the Yoast meta description for Android devices on the front/home page.
 */
add_filter( 'wpseo_metadesc', function( $description )
{
    return 
        ( is_front_page() || is_home() ) 
        && isset( $_SERVER['HTTP_USER_AGENT'] ) 
        && false !== stripos( $_SERVER['HTTP_USER_AGENT'], 'android' )
        ? 'Some Android description!'
        : $description;

}, PHP_INT_MAX );