BuddyPress.org

– login to your phpMyAdmin
– choose database where your WP/BP site is installed
– find and click on yourprefix_users to open
– scroll down to field user_login under field ID 1 (that’s you) and change to another username
– click on save and close phpMyAdmin

– you could change password at the same time, but it has extra steps seen here
https://codex.wordpress.org/Resetting_Your_Password#Through_phpMyAdmin