1. Launch Google Home application.
Go to Settings.
2. Click button to right of Devices, groups & rooms.
3. Go to the room to remove.
4. Tap Delete room.
5. Repeat for other Mesh devices.
6. Remove the wifi network. (There is no way to do this on Google Home.)

My experience:

It is either impossible or unlikely to succeed if you perform these activities on some spurious network to get a jump on your final installation. Simply do exactly as outlined here. In particular, you must use the ISP's actual modem/router hardware (and not some random switch somewhere else).

Here are the formal steps:

1. Need
   • Google Account
   • Google Home application installed on phone
   • Internet service
   • the ISP's modem
2. Set up a home in the application.
3. Set up first device.
4. Place device near modem.
5. Connect power cable to Mesh point.
6. Connect Ethernet cable to Mesh point and to modem.
7. Plug in power cable from Mesh point. (Light on Mesh point will pulse blue ready for set-up.)
8. Open Google Home application and press Add and manage.
9. Tap Set up new devices in your home.
10. Select your home by name. (App begins looking for devices.)
11. Accept the device found. (Connects to wifi router...)
12. When prompted, point phone camera at Q-code on bottom of Mesh point.
13. The app will finish the job.
14. Name the wifi network; choose a password easily remembered or written down.
    (When finished, Mesh point light will turn white.)

Connecting additional points...

1. Connect power cable to auxiliary Mesh point, then plug into power.
2. App will find the new device.
3. Follow the app's process.
4. Repeat for third device.

These steps are more or less a transcription of part of the video at How to Install and Set-up Google Wifi.