If your Confluence is blocked from accessing the Internet (e.g. for security reasons), the “Plan your upgrade” feature that’s built into Confluence will go out of date the moment a new iteration of Confluence is released. That’s bad if you are, you know, planning your upgrade.
The solution is to download a new version not of Confluence, but of the Confluence “Troubleshooting and Support” plugin. It’s free and you’ll find it on the Atlassian Marketplace here.
Once installed, don’t look for it under “Troubleshooting and Support tools” on the Confluence Admin screen (I know, right). Look for it under the Administration subsection in the left-hand menu between “Cache Management” and “License Details”:
Now you’ll be able to determine the next version of Confluence you should be upgrading to, and you’ll get step-by-step instructions on how to do it.
Tip: Atlassian support generally suggest that if you’re going to do a major version upgrade, first do an interim upgrade to the “*.0” version, e.g. v7.0. Just remember to upgrade quickly after that, especially if the interim version is subject to security vulnerabilities.