If you ever forget your admin password, hopefully, you have someone else who is an administrator that can reset it for you. If that is not an option, then the worst-case scenario is that you can log into the server that is hosting your NextCloud and reset the password via the command line with 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

The "www-data" user is going to be the user you have setup as your web service that run NextCloud. If you followed my post about setting up NextCloud, or if you are running on RHEL/CentOS/Fedora, that that user is going to be "apache". 

If your Nextcloud username is not "admin", then substitute the username that you setup as your Nextcloud admin.