Sony Ericsson W850i lens replacement?

Could you tell me how to replace the front glass (lens). The glass on mine is very badly scratched. I have bought a new lens but can't see any lugs or attachment points on it. I can't figure out how it actually attaches to the phone. Do I have to open up the phone to change it? Is it stuck on with glue? I would just like to be sure before I proceed. I have looked at http://www.formymobile.co.uk but it doesn't help.