You could (and probably should) wait for the wide release this fall before downloading iOS 8. But if you absolutely can't wait, here's how to get Apple's brand new iOS 8 beta on your phone right now.
Before you do anything, backup your device. It's a good idea anyway before updating, and considering how temperamental betas tend to be, it is especially important now. We should note too that we really don't recommend you do this unless you're a serious iOS addict. It's going to take a while for iOS 8 to be anything other than a buggy, volatile mess, and unless you have been scratching your eyes out in anticipation, you're going to regret it.
That said, if you absolutely can't wait (you can!), you have a couple of options.
If you're a developer: You can head straight over to Apple's Dev Center to lock down your copy. Just look for the model number on the back of your phone or iPad, and click the corresponding link.
If you're not an Apple dev: Or if you just don't want to pay the requisite $99 to become one, there are significantly cheaper options. All of them amount to essentially buying slots on already-made developer accounts, which is inherently sketchy and unreliable and you really probably shouldn't do it, but if you are desperate, there you go.
According to Gotta Be Mobile, IMZDL is one the best known and most reliable, which should have the beta available for download here. Still, it's kind of a gamble. And again, you almost definitely don't actually want to do this.
Once you have the beta downloaded:
- Plug your phone into you computer and connect to iTunes.
- With the device window open, hold alt/option for Mac and shift for Windows while you click Restore.
- Select the .ipsw file you just downloaded.
- Restore from your backup after finishing the installation
Oh, one last note: You really don't want to do this. Unless you're a dev. Really. But you are your own unique snowflake, and you have been warned. Happy trails!