How to change admin username and password?

You can’t change a username but you can create a new ‘admin’ user, then delete the OLD admin user and assign all the posts from the OLD user to the NEW user.

Just in case you need this: Resetting Your Password

John,

you might have solved this by now – but I got stuck with this myself (the other solutions mentioned here didn’t work for me) and hunted down a solution, then wrote it all down in a blogpost – so in case this helps:
https://graphiceyedea.co.uk/wp/2008/07/31/resetting-wordpress-admin-password/

Hi

yes you can change the admin username and password very easily via the wordpress database. Have a look at this video tutorial of how to do that: https://www.softsweneed.com/2009/04/03/41/

And it’s very good for security to change the default admin username

I agree with softsweneed, I change my username from the database as well. From there you can even change the wp_ prefix that comes as default on wordpress installations (which is suggested for security reasons).