Firstly, backup your existing save (just to be sure!). PlayerProgress.sav can be located in Steam\steamapps\common\Mini Motor Racing\Mini Motor Evo_Data. Load the game and once at the main menu, hold ALT+SHIFT+K together. You will be prompted with a dialogue confirming your desire to wipe ALL of your data - this includes Career progress, Cash, Car unlocks and Upgrades. If you select Okay, the game will quit and delete your Steam cloud save. Once you load the game again you'll back back to a fresh install.
If you ever want to return to your original save, quit the game and simply copy your backed up save into the directory above and over-ride the new one. When you launch the game again, Steam will ask which save you wish to use, cloud or local. Select the local save and you'll be back to where you where.
PLEASE NOTE: You will loose your Steam Achievements if you do this and they will NOT restore with your original save. ALT+SHIFT+K will remove them permanently, although they can be unlocked again through regular play.