Re: I see that there is a new version of blue ocean plugin. How do I upgrade?
This is a topic for the Jenkins users list, not the Jenkins developers list. The Jenkins developers list is for the development of Jenkins. Questions related to plugin upgrades and Jenkins administration are better handled on the Jenkins users list.
If you know which plugin hpi files you uploaded and have the earlier copies, you should be able to replace that plugin with the earlier version and restart your Jenkins server. Then you can use the Jenkins Plugin Manager to perform the upgrade.