To download the test version, select one of the download links below (depending on your system):
https://www.artsoft.org/RELEASES/linux/ ... rc1.tar.gz (Linux)
https://www.artsoft.org/RELEASES/mac/ro ... .0-rc1.dmg (Mac)
https://www.artsoft.org/RELEASES/window ... -win32.zip (Windows / 32 bit)
https://www.artsoft.org/RELEASES/window ... -win64.zip (Windows / 64 bit)
https://www.artsoft.org/RELEASES/androi ... .0-rc1.apk (Android)
You can also play and test it directly in your web browser:
https://staging.artsoft.org/rocksndiamonds/play
Upon first start of the release candidate version, and if you already have some R'n'D tapes stored on your system, the game will ask you if you want to upload them to the high score server. If you don't want to do that, or if you want to do it later, just say "no" here -- you can still upload your tapes from the setup menu as long as you haven't done it yet.
In addition, there are two new options related to the high score server in the setup/game menu to enable/disable using the score server, and to only show your own, local scores in the Hall of Fame (just like before).
If the score server should be unavailable for whatever reason (no network, server down etc.), you probably won't notice this immediately, as there will be no "timeout" delays (communication to the server is done in the background), and because Hall of Fame entries from the server will be cached locally -- the game will just silently disable any further attempts to access the server until next start (well, you will see an error message in the log file or on the command line).
To be able to rename your player name as shown in the Hall of Fame at a later time also for already uploaded scores, a randomly created unique, but anonymous player ID is generated. Just rename your player name in R'n'D, and the name shown for your score entries will be renamed, too.
To not start with an empty high score server, I have already uploaded the solution tapes from my tape test suite -- that's more than 25'000 score tapes from over 370 level sets, played by over 70 players. So you will already see some high scores for many level sets.
To have a look "behind the scenes", you can also take a look at the web interface of the high score server:
https://api-test.artsoft.org/
The site will show all approved scores (but not those that were rejected, most probably either because the level set is not yet known to the score server, or simply because the uploaded tape was a duplicate of an already existing tape, was not valid or was not solvable for whatever reason).
If everything works as expected, the final version 4.3.0.0 will hopefully be officially released soon, but I still expect a few hiccups here and there, as every attempt to release such a test version in the last few weeks resulted in stumbling upon yet another problem. But it seems to start running a bit more stable and reliably now (fingers crossed ... the last problems were fixed a few minutes ago, and my "issues" list is still not empty yet).
Future versions of the game and server (but not version 4.3.0.0 yet) may contain a way to replay score tapes that were uploaded and approved on the server from the Hall of Fame (directly or as some sort of cheat mode; not sure yet).
I hope you have fun testing the new pre-release version, and please let me know about everything good or bad that you should experience while testing the new version!
So it will be technically possible to choose an already existing name by intention and submit scores for it, which cannot be distinguished from the scores of other players with the same player name in the Hall of Fame (but can be distinguished on the server by their different player IDs). The only visible hint will be that there is more than one entry in the Hall of Fame with the same player name (as the server will only return the best score for each player for a given level). But it might be a good thing to at least let a player know that somebody else is already playing with the same nickname before choosing "John" as your player name. 
