Downgrade Jenkins

I easily rolled back to the last working version bydoing below steps:

1. Go to the location where jenkins is installed.
2. Rename jenkins.war if it exists to jenkins.war.corrupted
3. You would find a file jenkins.war.bak in same direcotry. Rename it to 'jenkins.war'

Downgrading should be equivalent to the upgrading process:

To upgrade from earlier versions of Jenkins, simply redeploy the WAR file. Read this document for more about container-specific instructions on how to do this. See this document for automation.

In my case, I followed the process below -

Download the jenkins.war file from https://updates.jenkins-ci.org
Stop the Jenkins service
Replace the jenkins.war with the new.
Restart the service.
Done

I have been struggling with downgrading Jenkins as well, lately because the latest version seems to have a problem handling JDK 5 code (which I am unfortunately tied to for a little while longer)

Here's how I did my downgrade:

1. Locate an earlier stable version at https://updates.jenkins-ci.org/
2. Download the war file
3. Stop the Jenkins service
4. Replace the jenkins.war with the new.
5. Restart the service.

I had an additional issue with the configuration file not being compatible. E.g. In my case I had to disable matrix to get the service running. Your case may be different, so I suggest another question.