cd /path/to/owncloud sudo -u webserveruser php occ user:resetpassword adminaccountnameIn our case the command was:
cd /var/www/cloud sudo -u www-data php occ user:resetpassword adminIt should prompt you for your password twice:

Websites and SEO
cd /path/to/owncloud sudo -u webserveruser php occ user:resetpassword adminaccountnameIn our case the command was:
cd /var/www/cloud sudo -u www-data php occ user:resetpassword adminIt should prompt you for your password twice: