Update from NexT v5.1.x

There are no hard breaking changes between 5.1.x and 6.0.x versions. It's change major version to 6 because:

Main repo was rebased from iissnan's profile to theme-next organization.
Most libraries under the next/source/lib directory was moved out to external repos under NexT organization.
3rd-party plugin 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:

You don't touch old next dir and just do some copies of NexT files:
1.1. config.yml or next.yml (if you used data-files).
1.2. Custom CSS styles what placed in next/source/css/_custom/* and next/source/css/_variables/* directories.
1.3. Custom layout styles what placed in next/layout/_custom/*.
1.4. Any another possible custom additions which can be finded by compare tools between repos.
Clone new v6.x repo to any another directory instead of 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.
Go to Hexo main config and set theme parameter: 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.