@Bernard Hi.
You can do one of the following:
Clear the app data from Android settings (settings - apps - GUI-O - clear all data). Note that this will clear everything, even stored devices.
Follow this link on GUI-O website:
https://www.gui-o.com/examples/gui-o-and-boards/esp32#h.7e5a31l59xj
Download the example and check the credentials in the code near the top. You can use port 8883 for secure communication, or 1883 otherwise. Client id can be empty, in this case GUI-O selects a unique generic client. You can also set it to your unique username.
Best regards,
Kl3m3n