Here are Apple's instructions on how to update, from Apple's support page:

Before you update

Make a backup of your device using iCloud or iTunes.

Update your device wirelessly

If a message says that an update is available, tap Install Now. You can also follow these steps:

1. Plug your device into power and connect to the Internet with Wi-Fi.

2. Tap Settings > General > Software Update.

3. Tap Download and Install. If a message asks to temporarily remove apps because iOS needs more space for the update, tap Continue or Cancel. Later, iOS will reinstall apps that it removed. If you tap Cancel, learn what to do next.

4. To update now, tap Install. Or you can tap Later and choose Install Tonight or Remind Me Later. If you tap Install Tonight, just plug your iOS device into power before you go to sleep. Your device will update automatically overnight.

5. If asked, enter your passcode. If you don't know your passcode, learn what to do.

If you see an error or need more space when updating wirelessly.

If you see an error message while trying to update your device wirelessly, learn what to do. If you need more space for a wireless update, you can update using iTunes or delete content manually from your device. Whether you use iTunes or update wirelessly, you'll have the same amount of free space on your device after you update.

Some iOS software updates aren't available wirelessly. VPN or proxy connections might prevent your device from contacting the iOS update servers.

Update your device using iTunes

If you can't update wirelessly on your iOS device, you can update manually using iTunes on a computer that you trust. If your computer is using Personal Hotspot on the iOS device that you're updating, connect your computer to a different WiFi or Ethernet network before you update.

1. Install the latest version of iTunes on your computer.

2. Connect your device to your computer.

3. Open iTunes and select your device.

4. Click Summary, then click Check for Update.

5. Click Download and Update.

If you see error messages in iTunes or need more space to update

Get help if you see error messages while updating your device with iTunes. Or if you need more space for an update, you can delete content from your device manually.