You can install iOS 12 using two ways. Either from within the device or using iTunes. Just go to:
Settings – General – Software Update
If you want a fresh install or your device is Jailbroken then you need to use iTunes to update your iPhone or iPad to the latest iOS 12.
How to Install iOS 12 Using iTunes
- Launch the latest version of iTunes on your computer
- Connect your iPhone or iPad using the lightning cable
- You will be instantly be prompted to download and install iOS 12