Hi there, welcome to the Community!
You can certainly update plugins, including the plugin manager itself, when the internet isn't available from your Confluence server.
We've got some steps on doing this in a KB article, which I've copied here for your convenience:
If not connected to the Internet or in certain troubleshooting scenarios, you may need to update the software by uploading the UPM JAR file to the Atlassian application. You need to follow these steps to update the Universal Plugin Manager for JIRA applications, Confluence, FishEye or Crucible from versions 1.5 and earlier.
To update the UPM manually:
- Download the latest version of the UPM software from the Atlassian Marketplace.
- From the top navigation bar in your application, choose > Add-ons or Manage apps.
- Choose Upload add-on or Upload app.
In the file chooser, browse to and select the UPM JAR file that you downloaded, then choose Upload.
A dialog appears indicating that the installation is being performed.
Depending on the UPM version you are replacing, it's possible for the UPM update to appear to fail. This may take one of several forms: the install progress dialog may appear to hang, or it may close quickly, or it may finish and show an error message. In most cases, the installation actually succeeds, but appears to fail due to issues with older UPM versions. If you think you have encountered this situation, wait a few minutes for the installation to finish and then refresh the browser. Check the footer at the bottom of the page to verify the UPM version.
- When prompted, refresh the browser or restart the application.
The UPM version is now up-to-date.
You can download other Server apps the same way - free apps should be immediately downloadable from their Marketplace pages, and you can get to the downloads for Paid apps if you need from the Versions option on their marketplace listings.
Let me know if you need any additional help!
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