[AC Question]

When I switched on my wi-fi connection the system update automatically started and I can't get it to stop. Someone please answer asap it's already at 28%

[B. Diddy]

Welcome to Android Central! Which device? It's probably too late--you never want to stop an installation midway, unless you want to brick the device.

[Mooncatt]

It depends on the phone. Sometimes if you can figure out which app is downloading the update, you can force close it from the app manager. If it's downloaded but not yet going through the installation, try restarting the phone into recovery and delete the system cache. This may delete the installation file.

Those methods are only a temporary fix, though, and it'll try downloading again. Most likely the only total fix will require rooting, then changing a system file to prevent it.