There is no special button.
On the GL.iNet Internet page, click “Scan” in the Repeater section, as if you are going to connect to another SSID. When the page comes back with the wifi list, click the SSID listbox and scroll down to the bottom to select “Other…”. You will then be able to manually enter the new SSID, Wi-Fi Security, Password and enable “Remember”.
Hopefully, when you click the “Join” button, it will save the new information without changing/disconnecting from the existing SSID.
Make sure that Repeater Options - “Auto scan and re-connect” is Enabled.