Update Jenkins from a war file
You can overwrite the existing jenkins.war file with the new one and then restart Jenkins.
This file is usually located in /usr/share/jenkins.
If this is not the case for your system, in Manage Jenkins -> System Information, it will display the path to the .war file under executable-war.
If you have installed Jenkins via apt-get, you should also update Jenkins via apt-get to avoid future problems. Updating should work via "apt-get update" and then "apt-get upgrade".
For details visit the following URL:
https://wiki.jenkins-ci.org/display/JENKINS/Installing+Jenkins+on+Ubuntu