You need to transfer those changes to a child theme before you can update.
However, if you’re already stuck to that version do to massive changes, you could just adjust the theme to work with WP 4.4.
Search for and replace occurrences of .accordion h3 with .accordion h2 in Nirvana’s admin/css/admin.css file and adjust the jQuery( “#accordion” ) call in admin/js/admin.js to use header: 'h2'