If you need to reset the password of a Jenkins Continious Integration server.
Stop Jenkins:
usually /etc/init.d/jenkins stop
edit the config.xml
usually: /var/lib/jenkins/config.xml
Replace
true
New
false
Possibly remove the Authorization configuration
Restart the Server:
usually /etc/init.d/jenkins start
Now your Jenkins is absolutely insecure, you can just go to the web interface and edit users password.
Do not forget to change back the security configuration, this could be done by a config.xml backup or through the web interface.