I want update my phone to the latest version

You haven't stated which model of iPhone you are using or which version of iOS you are running. However, the following might be of help:

Updating your iOS

Go to Settings > General > Software Update. You will be notified if your iOS is up-to-date or if there is an update available. 

Before following the instructions to download and update, you need to make sure that: 

Your battery is charged to at least 50%;

Low Power mode is not switched on;

You have a fast and strong Wi-Fi connection. (Updating over cellular data or a cellular hotspot is not possible);

You are not connected through VPN;

There is no beta profile installed from a previous version of iOS;

You have at least 8GB of storage free; and that

Your iPhone is connected to a charger throughout the entire operation. 

Or you can Turn on automatic updates:

Go to Settings > General > Software Update.

Tap Automatic Updates, then turn on Download iOS Updates.

Turn on Install iOS Updates. Your device will automatically update to the latest version of iOS. 

You still need to ensure that all the requirements listed above are met. 

[Some updates might need to be installed manually]

Update iOS on iPhone – Apple Support (UK)