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The version of Office Pro Plus that you receive with your E3 trial plan aren't upgrade versions, they're the full Office suite, the 2010 version. You'll uninstall your old copies of Office and install it whole cloth. If you're having trouble downloading, I'd suggest trying using a different web browser to get started, but you'll want to install them clean as your new copies of Office.
No version of Office is upgradeable to Professional Plus Subscription version. If you are installing the Subscription version all previous versions must be uninstalled.