1. Back up your device using iCloud or your computer.
2. Plug your device into power and connect to the internet with Wi-Fi.
3. Go to Settings > General, then tap Software Update.
4. If you see more than one software update option available, choose the one that you want to install.
5. Tap Install Now. If you see Download and Install instead, tap it to download the update, enter your passcode, then tap Install Now. If you don't know your passcode, learn what to do.

1. Go to Settings > General > Software Update.
2. Tap Automatic Updates, then turn on Download iOS Updates.
3. Turn on Install iOS Updates. Your device automatically updates to the latest version of iOS or iPadOS. Some updates might need to be installed manually.

Rapid Security Responses deliver important security improvements quicker, before they are included in future software updates.

To get Rapid Security Responses automatically:

1. Go to Settings > General > Software Update.
2. Tap Automatic Updates.
3. Make sure that Security Responses & System Files is turned on.

If you don't want Rapid Security Responses to be installed automatically, you can install Rapid Security Responses like software updates.

If you need to remove a Rapid Security Response:

1. Go to Settings > General > About.
2. Tap iOS Version.
3. Tap Remove Security Response.

You can reinstall the Rapid Security Response later, or wait for it to be installed permanently as part of a standard software update.  

For more information visit this article on Apple's site.