1. Download the "TSW2 - Livery-Manager" via the link in the forum post
2. Extraxt the downloaded ZIP somewhere and open the "TSW2LM.exe"
3. Folow the installation instructions
4. Select your Game directory (usually C:\[USERNAME]\Documents\MyGames\TrainSimWorld2\)
5. Select the folder where you extracted the downloaded BR423_unmodernisiert.zip
6. Press the huge "BACKUP!" button
7. Select a free spot on the right and then select the "BR423_unmodernisiert.tsw2liv" livery on the left
8. Press "Import"

Done!