Change "Read more" text in blog

You can of course edit all loop files that are affected and upload them as child-theme files.
Unfortunately, this is quite a lot of places to change, due to the fact that we have different cases here.
(Business-, Elegant-, Business-modern Blog) and that in loop-index.php, loop-portfolio-single.php etc.

but you can try this substitution via gettext:

function my_text_strings( $translated_text, $text, $domain ){
switch ( $translated_text ){
    case 'Read more'    :  $translated_text = __( 'Read more of it', $domain ); break;
     // in case you got different languages on your page and want to change them too:
    case 'Weiterlesen'  :  $translated_text = __( 'Mehr über das Thema …', $domain ); break;
  }
  return $translated_text;
}
add_filter('gettext', 'my_text_strings', 20, 3);

Yes for child-theme functions.php
and this second line was only meant to show how further substitutions have to be set. Important is the break setting after each line.
– However, if you had asked for a different language translation ( f.e. the german translation of the read more phrase) , I probably would have recommended editing the lang file. Again, you could find a child theme solution.