You can generate a new lockbox code to re-open the lockbox, get the keys again, and lock the car.
In the app, select the trip and tap on Get help > Generate lockbox code. Use that code to open the lockbox and get the keys. Lock the car, then put the keys back in the lockbox and shut the lockbox firmly.
Make sure the car is locked, the keys back in the lockbox, and the lockbox securely shut before you leave. Fees apply for leaving the car or lockbox unlocked.