You can use str_ireplace PHP function.
In the function.php file (hopefully in your WordPress child theme :-) insert these lines of code:
/*Replace read more in the blog section of WordPress*/
function newreadmore($newreadmore) {
$newreadmore = str_ireplace('read more', 'Click HERE To Read More', $newreadmore);
return $newreadmore;
}
add_filter('gettext', 'newreadmore');
I am using the 'gettext' filter: Read more here