Just before every patch, War Thunder opens it's development server for the public. Usually, this happens one or two times. Opening times are always announced prior by Gaijin.
But how do you get there? This is actually very easy! Just download the dev server launcher. Keep in mind, that you're basically redownloading the entire War Thunder game again. When the server opens, you can log into your regular account.
In-game, you have your regular silver lions and golden eagles balance. You have to unlock all vehicles that you do not yet have. There are 500% boosters constantly activated. You can play actual matches and earn RP and SL. Note: everything you earn or spend will not be transferred to the live server. The only thing that transfers is GE which you bought. GE you spend will not be transferred to your live account.
How to install the War Thunder Dev Launcher
Now you've grasped how the war thunder development server generally works it's time to download and install the War Thunder Dev Launcher from their official website, a good hint here to save you hours lost and a big headache... When you install the WT dev launcher make sure you don't save it into the same destination/location as your actual war thunder game.
You can download the launcher via this link: https://launcher.warthunder.com/wt_dev_launcher.exe
Once the dev launcher has been downloaded, click to open and wait a little bit for it to load.
If you're still facing issues installing the launcher you should first look through their Wiki/FAQ page, if that still doesn't help you're best off contacting their technical moderators via the forums.
And just like that, you have access to the dev server!