This is the beta firmware of iOS and it is not directly available under iOS update directly on your device.
iOS 12 Beta 8 iPSW Download for iPad and iPod
Make sure to backup your important stuff before installing the update for iPod or iPad.
iPSW Download Links
iPSW Download links for iOS 12 Beta 8 for iPad, iPod are live. Download the file compatible with your phone model. If any link is not working then inform in the comments. That’s all for now. If having any problems while installing iOS 12 Beta 8 for iPad/iPod then drop down comments. Share it and follow us on Twitter and Facebook.
For iPod
First of All download the iOS 12 Beta 8 for iPad or iPod ipsw firmware file from above links. Connect your iPad or iPod to the PC or Mac. Launch iTunes. Make sure to backup your data before installing the beta update. Now, if you are on Windows PC, hold “Shift” button and click on “Update”. If you are on Mac, hold “Option” button and click on “Update”. After clicking on Update, navigate to iOS 12 beta 8 ipsw file that you just downloaded and select it. The firmware will now start installing on your phone. When done, your phone will restart and you can disconnect from PC. iOS 12 Beta is now running on your phone.
By doing above steps you have installed the latest iOS beta firmware on your iPad.