Where do I update the registration form instructions?

I'm using Membership with BuddyPress, and would like to make the registration form a little more friendly, e.g. add guidance on what constitutes a valid username., and general advice as per the normal Membership registration

The registration form can be found here
http://ibsregister.com/register/?action=registeruser&subscription=1

Could you please tell me where I can find this and what the correct procedure is for updating it.

I have found what looks like the relevant code in plugins/buddypress/bp-themes/bp-default/registration/register.php

but changes made to this file are not reflected in the registration page.

Kind Regards
Johanna