1.x is no more supported and Jenkins 2.x is a major release. It is not as scary as it sounds though. The reason for the major version bump is a revamped UI and the merge of the Pipeline plugin (a DSL to define jobs).
Will first want to Upgrade Jenkins to 1.651.3 T144105
Upgrade process: https://wikitech.wikimedia.org/wiki/Jenkins
Upgrade guide: https://jenkins.io/doc/upgrade-guide/