How to make canonical on the main page?

Question

Landing pages

wordpress site.
Articles are displayed on the main page, and pagination is created page / 2, page / 3, etc.
The pages page / 2, page / 3 have canonical page / 2, page / 3, but how to make canonical for them on the main page.
What are the plugins or code. It's just that if you do a redirect, then the user will not see pages 2 and 3, etc.
And I need it purely for Google and Yandex robots.
And in the headings themselves should remain as is.

Answer 1

<?php if( is_paged() ){ ?>
  <link rel="canonical" href="<?php echo get_site_url(); ?>" />
<?php } ?>

Answer 2

When placed in the functions file, it gives an error
syntax error, unexpected '<', expecting end of file