How do I prevent my phone from restarting after a System Update?
When a system update is performed, the phone reboots to allow the new files to take over the old (the old files will be removed from the system). On some devices, it is possible to delay the update, but when it (the update) is done, one has to restart the device.

If you don't want to update at all, then install a firewall and block the wireless update app from accessing the internet.