To install any Euro Truck Simulator 2 beta update (taking the current ETS2 1.17 patch for example) simply go to your Steam library and select Euro Truck Simulator 2. Right click on the name then go to properties and then click on BETAS. Select the beta you want from the list (for example “public_beta – 1.17 public beta”). Hope this was helpful!
For more help, there is a tutorial of how to update at the beginning of this video: