How to reset/delete my glassfish password?

The content of glassfish3\glassfish\domains\domain1\config\local_password file is a backup password to log in as admin.

(let's pack some rep after the years)

You can also create a new domain, set the admin password there to whatever you like and move the password files over to your original domain. Here's a blog post explaining the process:

http://techteam.wordpress.com/2009/06/25/reset-glassfish-password/

With Glassfish 3.1.2.2, at least, you can replace the domain\config\admin-keyfile contents with the following:

admin;{SSHA256}UV9EsTaoxzAdtxiNMKTkb3g0WjnoUBEBgpDQIGSdOu8QhprFoWiSCw==;asadmin

This will give you a username of 'admin' and password 'changeit'.