This cannot be changed in the SilverStripe admin interface, but needs to be done directly in the PHP code. I'm not a PHP developer so I'm not sure how it works, but it appears to be defined in mysite/code/DownloadObject.php like so:
// used by Download page type
public static $typenames = array (
"win-x86" => "Windows",
"win-x86_64" => "Windows x86_64 (Vista or newer required)",
"mac-x86" => "Mac OS X (Intel)",
"mac-x86_64" => "Mac OS X x86_64 (10.8 or newer required)",
"mac-ppc" => "Mac OS X (PPC)",
"deb-x86" => "Linux - deb (x86)",
"deb-x86_64" => "Linux - deb (x86_64)",
"rpm-x86" => "Linux - rpm (x86)",
"rpm-x86_64" => "Linux - rpm (x86_64)",
"box" => "CD/DVD-images",
"src" => "Source code"
);
I can change the text there, but LibreOffice 5.4 is still supported and works with Vista I believe, so I don't want to change the text for all releases. And not being a PHP coder, I'm not sure how to make it detect whether it's talking about 5.4 or 6.0...