Look at the request window and expand the select box with the endpoint address. You should see something like this:
/
[edit current...]
[add new endpoint...]
[delete current]
You click on [edit current...] and you can change the value.
Here is an example using some available web service from w3schools.com:
http://www.w3schools.com/xml/tempconvert.asmx?WSDL
