Sometimes. Your order page lists what the network behind your own eSIM will accept, under "Add more to this eSIM". On an unlimited plan that is more days; on a fixed-data plan, more data.
It goes onto the eSIM already in your phone, so there is nothing new to install.
If the list is empty, this eSIM cannot be extended and a fresh plan is the way on. If the network turns a top-up down, you are not charged and we say so.