How can I update my iPhone?

You have two different questions that you are asking. The title of your post asks about how to backup your iPhone and that procedure is listed in this article published by Apple 👉 How to backup your iPhone.

The body of the post asks about how to update your iPhone and that procedure is listed below:

Make sure that your iPhone is setup to meet the following requirements before you attempt to update the iOS version over a Wi-Fi Network. Open Settings ➡️ General ➡️ Software Update: If there is an update available for your iPhone then it will be listed here. Follow the on-screen instructions to download and update your iPhone. 

• Your iPhone must have a fast Wi-Fi connection; you cannot update over cellular data or a cellular hotspot
• Ensure that you do not have a beta profile installed from a previous version
• The iPhone can not be in Low Power mode
• The battery state of charge must be above 50%
• Your iPhone must have at least 6GB or more free storage space
• Your iPhone cannot be connected through VPN
• You should leave the phone plugged in until it finishes updating, both to assure that it remains above 50% and to keep Wi-Fi enabled (Wi-Fi turns off when the phone sleeps if not connected to power).

Axel F.

Karimishaitu Said:

"I want to back up: how can I update my iPhone"

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Update Using a Computer:

Go Here:  Update your iPhone Using a Computer - User Tip