This will use the guerra24 server until you reboot, at which point it will reset to the openwrt one (there's no uci commit, so the server change not persistent).
uci set attendedsysupgrade.server.url="https://sysupgrade.guerra24.net"
owut upgrade ...
ERROR: Build failed with status 502 (--version-to 25.12.0-rc4 --device x86/64:generic:ext4)
The above errors are often due to the upgrade server lagging behind the
build server, first suggestion is to wait a while and try again.