Apple took the wraps off iOS 12 yesterday during its annual shindig for developers. In that announcement, it mentioned that the latest version of its mobile operating system is available from today - but only to some users. Here's what you need to do.
In order to get your hands on iOS 12 today, through legal channels, you'll need to sign up and become an Apple Developer. And then you'll need to stump up $149. While that's a little steep, there used to be separate developer fees for iOS and macOS which were $99 each. So the $149 isn't terrible value when you look at it that way. And it means you can get your hands on macOS Mojave as well.
Once that's done, go to the downloads section of the developer portal and download the provisioning profile for iOS 12. This identifies your device to Apple when it pings their update servers and lets them know the device is registered to receive beta versions of iOS 12.
Once that happens, the update will appear in Preferences, when you tap on the General section. For my iPhone X, the update is 2.61GB but that will vary depending on what device you have.
Then, you'll receive the update just like any other iOS update that's delivered over the air.
Based on past experience, Apple releases an update to the beta every couple of weeks so, if the first beta causes issues with your apps, you only need survive a couple of weeks before there's an update that should, in theory, be better than the previous beta.
A word of caution
We're talking about beta software. That means you shouldn't install this on a device you rely on.
I've played beta-version-roulette many times with previous releases of iOS and the first betas can be a tad flaky. You might find things don't work and random crashes and restarts aren't uncommon.
Also, make sure you do a full back up of your device. Don't rely on iCloud for this. Do it using iTunes so you're not dependent on the device connecting to the internet in order to restore your data.
And, if it all goes pear shaped, the good news is you can download a restore version of iOS from the same place so you can go back to a more stable release of the software.