themes/next/docs/UPDATE-FROM-5.1.X.md
There are no hard breaking changes between 5.1.x and 6.0.x versions. It's change major version to 6 because:
next/source/lib directory was moved out to external repos under NexT organization.hexo-wordcount was replaced by hexo-symbols-count-time because hexo-symbols-count-time no have any external nodejs dependencies, no have language filter which causes better performance on speed at site generation.So, i suggest to update from version 5 to version 6 in this way:
next dir and just do some copies of NexT files:config.yml or next.yml (if you used data-files).next/source/css/_custom/* and next/source/css/_variables/* directories.next/layout/_custom/*.next. For example, in next-reloaded directory: git clone https://github.com/theme-next/hexo-theme-next themes/next-reloaded. So, you don't touch your old NexT 5.1.x directory and can work with new next-reloaded dir.theme: next-reloaded. So, your next-reloaded directory must loading with your generation. If you may see any bugs or you simply not like this version, you anytime can switch for 5.1.x version back.And how to enable 3rd-party libraries see here.