Minecraft 1.12 recently came out and so obviously I am gonna have to update the game to get it. However, if I am connected to WiFi and I open Minecraft, it starts to update. It fills up the loading bar 120.38 MB. (all the way) And then it just resets at 0.00 MB. I can play in 1.12 if I turn off WiFi, but I want to play online on servers. Please help! Edit: After it does this about 3 times it displays the following message:Unable to update the Minecraft Native Launcher
I had that problem too for a couple months. It turned out that my account didn't have the permissions to update the launcher, because the launcher is a universal application over all the accounts and the minecraft versions are separate between accounts. If you go to an admin account and install it there, you should be good!
But if you're not, you're going to want to uninstall minecraft and reinstall it.