So far we don't have any downgrade version available for Lightroom 6 .
It just compatible to be installed on Windows 7 with Service Pack 1, Windows 8, Windows 8.1, or Windows 10.
Please refer to the system requirements for Lightroom 6 for more information : https://helpx.adobe.com/lightroom/system-requirements.html
Adobe accepts returns within 30 days of receipt. If purchased through a re-seller, requests for refunds and product return must be made via the original store or on-line retailer.