When you delete NASCAR Heat Mobile, or get a new update, you will see this message:
"A previous iCloud save game has been detected. Do you want to use the saved NASCAR game save or disable iCloud saves on this device?"
If you want to use the existing save game than you should select yes. If you want to start fresh, select no and disable iCloud saves.
Selecting no will delete all of your previous game progress and you will restart the game from scratch.