How do you update records in intermediate tables in CI3?

Tables are Posts and Tags. 

One Post has Many Tags. And One Tag has Many Posts. A so-called 'Many-to-Many' relationship. 

This is solved with a intermediate table (post_tag) with id for post_id and tag_id and a created_at row. 

Now, How do you handle this in CodeIgniter? 

And before you answer. I also wish to learn how to handle updating records in intermediate table. 

As in, I remove a Post or I add a new Tag to a Post. 

Thanks,