Welcome, Glynne, to the OEM forum.
Reset / reconnect emonPi to a different network:
To force an EmonPi back into AP mode, go into ‘WiFi’ from settings and deselect the currently selected WLAN. Click on save and then refresh. You will still have access to the emoncms instance. Navigate to the admin menu and click reboot. The EmonPi will now come up in AP mode (as no network is configured).
(from Force the Emonpi to AP-mode - #4 by borpin)
Then it’s essentially start again with setting up Wi-Fi.