Is there any guide I can look at where I can change the values Summary (getSummary in API) using the extensibility API?

I don't quite understand what you want to do, getSummary() just returns pre-defined values based on the status of the cart. These values are not changeable with code - they change depending on the contents of the cart.

Like changing or manipulating the values coming out from the getSummary. I just do not want lean towards the old way of extending the modules. I wanted to make most out of the extensibility API. Also, avoidning the manipulating the DOM

OK, well to answer this question like I did the other one: this is not something we allow with the extensibility API.

Thanks Steve!