Regarding changing your WordPress username, unfortunately, it is only possible to do so via MySQL's console. To load it execute the following command (when you get asked for your password, choose your main instance password):
sudo /opt/bitnami/mysql/bin/mysql -u root -p bitnami_wordpress
From there, you should have access to the MySQL console. Execute the following (but remember to change youradminusername to your desired username):
UPDATE wp_users SET user_login = 'youradminusername' WHERE ID = 1;
exit to go out from the MySQL console. Now, you should be able to access WordPress using your new username.
I hope it helps!
Carlos R. Hernández