A Confluence instance that is not connected to the internet cannot check for plugin updates, or compatibility of plugins with newer versions of Confluence.

It would be great to have a way for these instances to check for new plugin updates, and to check their plugin compatibility with new Confluence versions.

Workaround

Create a test server with access to the internet, using the data from the live server:

1. Set up a test instance with access to the internet, follow the instructions here.
2. Go to Confluence Admin > Plugins > Manage Existing, and see which plugins say that they require an update
3. Update them in the test system, making a note of which version they have updated to
4. Download the plugins manually from marketplace.atlassian.com
5. Upload the plugins to the live system