This is an updated thread replacing the previous ones that have been archived.
MX 6 only functions on Windows 7 or later.
At present (April 2021) it is not possible to change the User Agent in MX 6 using the traditional internal UA modifier/selector so you have to use extensions from the chrome store https://chrome.google.com/webstore/search/user agent?_category=extensions. The choice is yours.
The Qupzilla browser is now obsolete and is no longer being developed but it still functions well in the UA. If anybody wants to suggest a replacement please post it here. We can try them