u/Neloth-

Official steps in here didn't work for me. It creates a loop: you need to disable Steam Cloud, but the game won't let you start the game while Steam Cloud is off. So I find a way finally! If anyone tries it, please confirm it.

1. Disable Internet connection.
2. Go to C:\Program Files (x86)\Steam\userdata\[9 digit Steam ID]\1551360\remote\[folder name with 16 numbers] folder and delete files inside it.
3. Go to C:\Users\[User Name]\\AppData\Local\ForzaHorizon5\User_SteamLocalStorageDirectory\ConnectedStorage\User_XXXXXXXXXXXXX folder and delete two files: one named ProfileBackup in the end and one named Meta in the end.
4. Start the game and play a little bit and exit.
5. Open your Internet and start the game again.
6. Steam will ask which cloud save should be loaded. Choose local and start your new game.