You can restart Mario Kart Tour from scratch by deleting your Mario Kart Tour save data.
Note: Deleting your save data will also unlink your Nintendo Account. You will not be able to recover the data (rubies, coins, drivers, etc.).
Take the following steps to delete your Mario Kart Tour save data:
① Tap Menu at the bottom of the cup-select screen.
② Tap settings at the bottom right of the screen.
③ Scroll down to Other and tap Start Fresh.
④ Follow the on-screen instructions.