Should be in the Installer menu. But you'll need to connect the Sky Q boxes by Ethernet first otherwise they won't work.
There's a guide at https://www.trustedreviews.com/how-to/fix-sky-q-connectivity-problems-4099650 whch shows the process. Basically connect each box by Ethernet, reset the network on each Sky Q box so it detects the Ethernet connections, then turn off the WiFi in the Installer menu.
