Here is a how-to tutorial that will help you install iOS 8 Gold Master version on your iPhone 5s, 5c, 5, 4s, iPad 2 and over and iPod Touch 5G in simple ways.
Before we go deep into this installation guide, it’s important to note that iOS 8 GM is available only for developers. There are, however, no strict rules whatsoever. So, even if you are not an iOS developer, you can still download and install the latest iOS 8 GM on your devices.
Do you need a developer account for downloading iOS 8? Nope.
Do you need UDID registration for installing iOS 8? Nope.
What you need is an iOS 8 compatible device and time. And then you can install the new iOS 8 GM as quickly and easily as you choose to. However, we must repeatedly let you know that this firmware is strictly for developers. You can go ahead and try it on your device, but please be aware of the risks involved.
What risks? So far, we have faced nothing unusual. Apple, however, recommends consumers to “wait” for the iOS 8 stable version. Betas and GM are usually reserved for developers.
Now, enough of the lecture, and let’s get down to some serious business. Let’s explain to you in this guide on how you can download and install iOS 8 GM. We have two ways, each of them explained below:
Method #1: Install iOS 8 GM Through iOS Dev Center
Step 1: Login with your Apple ID and password at developer.apple.com/programs/ios/ — note that this is paid program that costs $99/year.
Step 2: Register your device. This step is simple. First, plug your device to iTunes and note down the UDID number. Copy this and paste it in the developer portal.
Step 3: Next step is to look for the iOS 8 download links. You can find them in the same portal, or you can also head over to our iOS 8 GM download page.
Step 4: The file will be in .zip format, so extract it to get IPSW file format.
Step 5: Launch iTunes and connect your iOS device.
Step 6: This step is important. Hold Alt key (on Mac) or Shift key (on Windows) and click on the “Restore” button from your iTunes.
Step 7: This opens a window wherein you have to navigate to the zip (IPSW) file you have just downloaded. Select this and wait for the installation to complete.
Method #2: Install iOS 8 GM Without Developer Account
Step 1: Connect your iOS device to computer and launch iTunes.
Step 3: Once you have downloaded the files, extract it to receive an iOS 8 IPSW file.
Follow step 5 to step 7 from the previous method to quickly install iOS 8 on your iPhones, iPads, and iPod Touch.
Source: iOS 8 Installation Guide