Oh, that's not good advice. Uninstalling the latest update to Flash won't remove Flash from the system, and it will leave you with a vulnerable, older version of Flash Player. Installing the ActiveX player for Win7 is not going to upgrade the Flash Player component that's built into Internet Explorer, so the user will still be stuck with an outdated version and no beta version until they run windows update again.
We don't distribute a beta version for Internet Explorer on Windows 8 and higher at all because we don't own that distribution channel (this is the same reason that we don't have a Chrome beta), and the Win8 binaries are different than what we release for Windows 7. There's effectively no way to do this at the moment.
We're talking to Microsoft about providing a mechanism for distributing betas and extended support releases of Flash Player to Win8+ users, but those distribution vehicles would have to come directly from Microsoft and we're still just talking about it, so there's no solution in the immediate future.
If you want to evaluate beta builds on Win8+, you're basically looking at NPAPI on Firefox and/or PPAPI on Opera.