Hi,

Sorry for the delay.

You can edit the reminder email content via wordpress filters. 

In your theme functions.php file add following filter:

add_filter('stec_mail_remind_subject', function($init_subject) {

    $init_subject = __("You have requested remind for stec_replace_event_summary", 'stec');

    return $init_subject;
});
add_filter('stec_mail_remind_message', function($init_message) {

    $init_message = __("You have requested remind for stec_replace_event_summary.\n\n"
            . "Event date: stec_replace_event_timespan.\n\n"
            . "For more info visit stec_replace_siteurl.", "stec");
    return $init_message;
});