using Ajax for link_to

Hello, it is my first question
I have some problem with implementation java script in my project
I have a lot of carts and a have a link add_quantity
and I need to increase the quantity on 25 each time

I made it without ajax but I need to use it....

I know that update method not correct...Is somebody has idea?

<td><%= link_to_remote 'add quantity', :url =>
add_quantity_cart_path(id: cart) , update: => cart.quantity(id: cart)

What version of Rails are you using?

Javier Q