How to using Co-Authors plugin with Zelle

To integrate Co-Authors Plus, you’ll want to replace existing author template tags in your theme with a simple conditional that uses the Co-Authors Plus template tags if Co-Authors Plus is available. The conditional prevents your site from breaking (e.g. white screen of death) if Co-Authors Plus isn’t activated.

You need to login to your FTP, and navigate to theme's /inc/ folder, and edit template-tags.php file. Replace the following block of code:

function zerif_posted_on() {
	$time_string = '<time class="entry-date published" itemprop="datePublished" datetime="%1$s">%2$s</time>';
	if ( get_the_time( 'U' ) !== get_the_modified_time( 'U' ) ) {
		$time_string .= '<time class="updated" datetime="%3$s">%4$s</time>';
	}
	$time_string = sprintf( $time_string,
		esc_attr( get_the_date( 'c' ) ),
		esc_html( get_the_date() ),
		esc_attr( get_the_modified_date( 'c' ) ),
		esc_html( get_the_modified_date() )
	);
	printf( __( '<span class="posted-on">Posted on %1$s</span><span class="byline"> by %2$s</span>', 'zerif' ),
		sprintf( '<a href="%1$s" rel="bookmark">%2$s</a>',
			esc_url( get_day_link(get_the_date('Y'), get_the_date('m'), get_the_date('d')) ),
			$time_string
		),
		sprintf( '<span class="author vcard" itemprop="name"><a href="%1$s" class="url fn n author-link" itemscope="itemscope" itemtype="http://schema.org/Person" itemprop="author">%2$s</a></span>',
			esc_url( get_author_posts_url( get_the_author_meta( 'ID' ) ) ),
			esc_html( get_the_author() )
		)
	);
}

And replace it with the following

function zerif_posted_on() {
    if ( function_exists( 'coauthors_posts_links' ) ) :
        printf( __( '<span class="%1$s">Posted on</span> %2$s <span class="meta-sep">by</span> %3$s', 'zerif' ),
            'meta-prep meta-prep-author',
            sprintf( '<a href="%1$s" title="%2$s" rel="bookmark"><span class="entry-date">%3$s</span></a>',
                get_permalink(),
                esc_attr( get_the_time() ),
                get_the_date()
            ),
            coauthors_posts_links( null, null, null, null, false )
        );
    else:
        printf( __( '<span class="%1$s">Posted on</span> %2$s <span class="meta-sep">by</span> %3$s', 'zerif' ),
            'meta-prep meta-prep-author',
            sprintf( '<a href="%1$s" title="%2$s" rel="bookmark"><span class="entry-date">%3$s</span></a>',
                get_permalink(),
                esc_attr( get_the_time() ),
                get_the_date()
            ),
            sprintf( '<span class="author vcard"><a class="url fn n" href="%1$s" title="%2$s">%3$s</a></span>',
                get_author_posts_url( get_the_author_meta( 'ID' ) ),
                esc_attr( sprintf( __( 'View all posts by %s', 'zerif' ), get_the_author() ) ),
                get_the_author()
            )
        );
    endif;
}

That should do. You can visit their official docs for more info.