First, you must be able to login with root privilege user on the server. Here, I will ssh into the server.
$ ssh server_admin@server_ip_address
Note that server_admin is the username for the server system (Ubuntu in my case) and server_ip_address is the IP address of the server.
Once you are logged in, you need to run
$ sudo -u www-data php /var/www/html/owncloud/occ user:resetpassword owncloud_admin
Here, I am assuming that you followed my previous post, so owncloud folder path is /var/www/html/owncloud. Replace owncloud_admin with your owncloud admin username, whose password you want to reset.