Hi,
Here is an article about the theme update – ps://xstore.helpscoutdocs.com/article/63-theme-update
6.3.6 is the latest theme version, so it is fine on your site. Please make sure that version of Core plugin is 2.3.6 (Dashboard > Plugins). If so, then “Add To Cart With Quantity ” should be available in Theme Options. To find out why the option doesn’t visible for you, we need to take a closer look. Provide temporary wp-admin access. Create an account via Dashboard (Users > Add new) with administrator role http://prntscr.com/s3rc9m > provide us with username and password via the Private Content area http://prntscr.com/s5mao7 You can use any email for it. Delete this account once all questions will be solved.
Regards