Update user data using web services

Hello everyone, 

I'm using moodle v2.0 and I'm trying to update a concrete user's data using a webservice in php. The main problem is that I've been looking for information through the internet and since I'm new at this, (moodle) I don't know exactly how to manage it.

I've been trying with conduit webservice, but I don't get exactly how to make it work.

The main question would be: If I want to update the username of a user using a webservice in php, which is the right (and easiest) way to do it? There is any tutorial easy to understand with this kind of examples?

Thank you very much.