Resetting a lost admin password
The normal ways to recover a lost password are:
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.
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
If your Nextcloud username is not admin, then substitute your Nextcloud username.