The easiest way is use trigger links. You can create a trigger link that adds or removes a tag. And then at the bottom of each email (normally in the footer), you can display those links. So the links could say something like:
- I no longer want to get emails about Topic A
- I want to get emails about Topic B
- I want to get emails about Topic C
To do it properly, you'd need a bit of liquid code, which would check what tag the subscriber currently has. I've written an article with an intro to liquid here11, which may help. There is also this ConvertKit help doc15 and this one on trigger links18.