Changing the_posts_navigation() html output

This function use the get_the_posts_navigation() to change the text Older posts and Newer posts you can pass array with the args.

See this example with the default values:

the_posts_navigation(
    array(
        'prev_text' => __('Older posts', 'theme_textdomain'),
        'next_text' => __('Newer posts', 'theme_textdomain'),
        'screen_reader_text' => __('Posts navigation', 'theme_textdomain')
    )
);