You don’t need to dalete theme. Only you need to delete which plugin you want to update latest version.
1. Deactivate when you need an update premium plugin/s
2. Delete the plugin first (which plugin you want to update) and Cosmos theme will be inform to you missing plugin/s again
3. install updated version of the plugin/s
Alo can you provide us you admin login? (you can simply reply this as private with your info site url, admin, pass) We’ll install it for your for free!