see: https://docs.nextcloud.com/server/20/admin_manual/configuration_user/reset_admin_password.html


Resetting a lost admin password

The normal ways to recover a lost password are:

1.    Click the password reset link on the login screen; this appears after a failed login attempt. This works only if you have entered your email address on your Personal page in the Nextcloud Web interface, so that the Nextcloud server can email a reset link to you.

:2.    Ask another Nextcloud server admin to reset it for you.

If neither of these is an option, then you have a third option, and that is using the occ command. See Using the occ command to learn more about using the occ command.

$ sudo -u www-data php /var/www/nextcloud/occ user:resetpassword admin
Enter a new password:
Confirm the new password:
Successfully reset password for admin