Google Nest does not sell spare parts, but you should be able to contact Google Nest Support for a replacement doorbell if it is within the 1-year warranty. The doorbell shouldn't just fall off of its mount (though other customers have also reported this).
To reach Support, you need to start with this link; depending on your location and time of day, you may have contact options:
It looks like we can consider this one complete, so I will be locking this in 24 hours if we won't hear back from you again. Feel free to start a new thread and we'll be happy to help.