Depends what you want to put in capital letters. If it was the student name for example it would be in https://github.com/mdjnelson/moodle-mod_customcert/blob/MOODLE_311_STABLE/element/studentname/classes/element.php#L46 and https://github.com/mdjnelson/moodle-mod_customcert/blob/MOODLE_311_STABLE/element/studentname/classes/element.php#L60. If you want everything capitalised then you could fiddle around in the methods 'render_content' and 'render_html_content' in the 'element_helper' class, see https://github.com/mdjnelson/moodle-mod_customcert/blob/MOODLE_311_STABLE/classes/element_helper.php.
Custom certificate (plugin)
Upper case in custom certificate
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