How do I change the modem code for a new modem

See following from Ring support https://support.ring.com/hc/en-us/articles/360034825092-How-to-Reconnect-Your-Ring-Device-to-Wifi-or-Change-Your-Wifi-Network

Check the Ring app

You can manually reconnect your Ring device back to your wifi network by following these steps:

• Open up the Ring app and tap on the three lines on the top left.
• Look for Devices in the list on the left side of the screen. 
• Tap Devices.
• Select the device (camera, doorbell, etc.) you need to reconnect to wifi.
  • Use the same process to change to a new wifi network.
  • Use the same process to reconnect multiple Ring devices to wifi.
• Tap on Device Health on the bottom of the screen.
• Tap on Reconnect to Wifi or Change Wifi Network.

Note: Once your device is reconnected to wifi, you can return to this screen to see your wifi Signal Strength under Network.