It’s easy to keep your iPhone up to date with the latest features and security enhancements. We’ll walk through how to update your device wirelessly and how to update it using your computer. Before you start, make sure you back up your device, plug it into power, and connect to Wi-Fi. If you’re using a Mac or Windows computer to update your device, you’ll need a compatible cable. We’ll start by showing you how to update your iPhone wirelessly. When there’s an update available, you might get a pop-up onscreen, which you can tap to start the update process. Or a badge might appear on the Settings app icon on your Home Screen. To check for an update, open Settings, tap General, and tap Software Update. You might have the option to choose between two available updates. If you’d like to continue using the version of iOS that’s currently on your device, you can still install important security updates for some time, or you can upgrade to the latest version.

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Now tap Download and Install. This button might read “Install” if your device has already downloaded the update. Then enter your passcode and follow any additional prompts. Your iPhone will then begin the update process. When the update is complete, your iPhone will restart. Now we’ll show you how to update your iPhone using your computer. Before starting the update, make sure your device is plugged into your computer and that your computer is connected to the internet. If you’re using a Mac with macOS Catalina or later, open the Finder. If you’re using a Mac with macOS Mojave or earlier – or a Windows computer — open iTunes. You might receive a notification when a new update is available, which you can click to start the update process. Otherwise, locate your device in the sidebar and make sure you’re in the General tab. Next, click Update. Then click Update again and follow the prompts. Now you know how to keep your iPhone up to date and can enjoy all the latest features. 

Source: How to update your iPhone | Apple Support