Some services, in particular user directories and gateways to legacy IM systems, require registration. To register with such a service, either type M-x jabber-get-register or select it from the Service menu, which is opened by typing C-c C-s. You have to know the service's JID, possibly from service discovery. (see Service discovery and browsing)
This is also the way to change your registration details, e.g. your password — just ask to register with that service again. To change the password of your Jabber account, ask to register with your Jabber server.
Please note that any passwords sent in this way will be sent in cleartext to your Jabber server, unless you have enabled SSL encryption (see Connection settings), and possibly sent in cleartext from your server to the server hosting the service.
jabber.el will then request a registration form from that service. If for some reason the service does not answer (maybe network problems, or some services neither support registration nor report errors about that) that will be the last thing you saw about it. jabber.el will not report timeout errors, but rather simply wait until you shut it down.
Once the response arrives, the form will be rendered in a browse buffer. Just fill out the fields, and hit Submit. You will receive confirmation of your registration in the echo area.
To cancel an existing registration (and also for cancelling your Jabber account, if you sent a registration request to your server), hit Cancel. The unregistration will be confirmed in the echo area.