1. Turn on Homey
Make sure Homey is upright and on. Make sure that Homey is on for at least half a minute before you continue.
2. Turn Homey upside-down
Keep Homey upside down, and wait for the 10-second countdown to finish. Homey is now in recovery, and will ask you to go to setup.athom.com on a computer, just like when you install Homey for the first time.
3. Open setup.athom.com on a computer
Go to setup.athom.com on a computer with WiFi and follow the instructions on the screen. Choose between clearing all data (restore to factory settings), or keeping the data and continuing to use Homey. With the wizard on the screen you can now (re) connect Homey to the WiFi network you want.