Edit pagination text in the get_the_posts_pagination function

You can modify screen_reader_text argument when invoking the_posts_pagination() wrapper function in your theme files:

<?php the_posts_pagination( array(
    'mid_size'  => 2,
    'prev_text' => __( 'Back', 'textdomain' ),
    'next_text' => __( 'Onward', 'textdomain' ),
    'screen_reader_text' => __( 'Whatever', 'textdomain' ),
) ); ?>

Search for the_posts_pagination in your template files, and adjust texts as you wish.