When you setup seahub website, you should have setup a admin account. After you logged in a admin, you may add/delete users and file libraries.
Administrator can reset password for a user in "System Admin" page.
In a private server, the default settings doesn't support users to reset their password by email. If you want to enable this, you have first to set up notification email.
You may run reset-admin.sh script under seafile-server directory. This script would help you reset the admin account and password.