If your Jira instance cannot access the Marketplace, you can download the last version here. Upload it through the Atlassian Universal Plugin Manager in the Jira Administration section: Administration → Manage apps → Upload app.
To enable Jira to upload and install a .jar file, please make sure to use the correct JVM flag.
Here is the referring documentation: https://confluence.atlassian.com/jirakb/how-to-re-enable-plugin-upload-1364557898.html
