Go into your Steam library and right-click on rFactor2/properties/beta's/opt-out (no beta). The new UI should be activated automatically.
Maybe you have to wait for the download to complete after selecting 'no beta'. I'm sorry, but my French isn't too well, so I don't understand the message in that picture.
that's it it works! in fact you had to start Steam in administarteur mode. On the other hand this interface ... difficult to make more complicated ... (and no French, the total! Lol) well will get used to it but frankly for me it's wasted time devoted to that there is good better to do before to further improve RF2. Thank you for your advice and best wishes for 2021