Can anyone explain how to use webservice in existing application?

Hello friend its depends on your flow of application, means which type of architecture you follow.

but simply you can try this to access your webservice method using this normal steps..
1 Way...

1) add web reference of your web service to your application
2) Create a Object of your webservice reference in your code behind page.
3) now simply call your webmethods using that objectname.(dot) your method name
or if your method is getting parameters then pass it to it when you call it.

2 Way...

You Can also call your web method using jQuery and Ajax call
How to call a Web Service from client-side JavaScript using ASP.NET AJAX[^]
Calling Web service[^]