You will find 3 authors within our company blog, each author has website url in profile configurations:

Mike - http://mike.com
Gelens - http://gelens.com
Admin - http://site.com/company/

the hyperlinks for profiles are:

http://site.com/author/Mike/
http://site.com/author/Gelens/
http://site.com/author/Admin/

I have to replace a hyperlink to Admin's page, so, if there's <?php the_author_posts_link(); ?> tag on some page, and also the author is Admin, the hyperlink should be http://site.com/company/ rather than http://site.com/author/Admin/.

How do i do this?

That's URL spinning with .htaccess, that is possible by editing the .htaccess manually.

But simpler for any beginner having a wordpress plugin for example http://wordpress.org/extend/plugins/redirection/ which appears enjoy it is going to do the thing you need.

It appears such as the the_author_posts_link function just calls get_author_posts_url to obtain the link, which passes the hyperlink with the author_link filter before coming back it. Inside your theme's functions.php, you could include something similar to this (untested):

add_filter( 'author_link', 'admin_author_link', 10, 3)

function admin_author_link($link, $author_id, $author_nicename) 

    return $link

I believe minimal acid reflux could be wordpress > the_author_meta.

Have each user add their url within the wordpress account, as you've done. Then inside your theme's functions.php use the_author_meta('user_url'). Don't forget this will echo the url. For doing things like a variable use get_the_author_meta('user_url').

This is how we made it happen using the twenty ten theme, this really is in functions.php

function twentyten_posted_on() {
printf( __( '<span class="%1$s">Posted on</span> %2$s <span class="meta-sep">by</span> %3$s', 'twentyten' ),
    'meta-prep meta-prep-author',
    sprintf( '<a href="%1$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_the_author_meta('user_url'), //changed from get_author_posts_url( get_the_author_meta( 'ID' ) ),
        sprintf( esc_attr__( 'About %s', 'twentyten' ), get_the_author() ),
        get_the_author()
    )
);
}

This can be done using http rewrites.