Wordpress - Change author permalink

you need 3 simple functions and hooks

first change the author base:

//change author/username base to users/userID
function change_author_permalinks() {
  global $wp_rewrite;
   // Change the value of the author permalink base to whatever you want here
   $wp_rewrite->author_base = 'users';
  $wp_rewrite->flush_rules();
}

add_action('init','change_author_permalinks');

then add users to query_vars:

add_filter('query_vars', 'users_query_vars');
function users_query_vars($vars) {
    // add lid to the valid list of variables
    $new_vars = array('users');
    $vars = $new_vars + $vars;
    return $vars;
}

then add the new rewrite rule:

function user_rewrite_rules( $wp_rewrite ) {
  $newrules = array();
  $new_rules['users/(\d*)$'] = 'index.php?author=$matches[1]';
  $wp_rewrite->rules = $new_rules + $wp_rewrite->rules;
}
add_filter('generate_rewrite_rules','user_rewrite_rules');

Now if you don't know how to use this, just copy all of the code and paste in your theme's functions.php file.