How to get definition of web service which we are consuming in our project.

Hi Experts,
I have added web reference in my project and using that i am consuming one method in my project but I need to change definition of that method due to some requirement. Can i do this with existing web service or i have to create new one with my new definition. I want to go to the definition of web service which i am consuming in my project.
please help me.
Thanks in advance.