I have yet to find a way to shut off updates in Win 8.1. What you can shut off are app updates but not OS updates - and when it decides to update you can not turn off the computer without letting the update download and install. I just went through this at a campground with limited wifi. If anyone can specify where to set a setting to stop this from happening please share details.
As to updating to Win 10, make a full drive backup first just in case the update stinks worse than 8.1 does.