There are many ways to update iPhone. The easiest way is to update via air. Certainly, iTunes is another great choice. Just follow below steps.
Update iPhone to iOS 12 via Air
1. If a pop-up panel shows you that iPhone latest version iOS 12 is available, you can click on the "Install Now" button to start to update iPhone to iOS 12.
2. It is also accessible to check out if iOS 12 is available manually now. The following steps will instruct you to make it.
a) Connect iPhone to a good Wi-Fi network and make sure your iPhone storage is enough. You also need to ensure that the battery of your iPhone can support the update.
b) Click on "Settings > General > Software Update> Download and Install" successively and iPhone will start to download and install iOS 12 right away. Wait until the update is completed.
Update iPhone to iOS 12 via iTunes
1. In order to use iTunes to update iPhone to iOS 12, you can first download iOS 12 IPSW file from this website (click here).
2. Run the latest version of iTunes on computer and insert iPhone to computer.
3. Click on iPhone icon on iTunes interface and then hit "Summary> Check for Update".
4. Choose the IPSW firmware file that you have downloaded beforehand by performing the following actions in iTunes: >
Mac: OPTION + Click on the "Update" button in iTunes
Windows: SHIFT + Click on the 'Update' button in iTunes
5. Navigate to your computer folder to choose the IPSW file and then click on "Update" button to confirm that you would like to install iOS 12 in the pop-up panel.
6. Wait until the update is over and iOS 12 is installed on iPhone. In the end, your iPhone will reboot with iOS 12 successfully installed.
For more details, check: How to Update iPhone to Latest iOS 12