How to update IOS on iphone if I don't have a laptop?

I have a laptop but I don't have the USB cable that connects iphone to laptop.

You don't need a laptop or cable, just go to Settings / General / Software Update.
But how do you charge the phone if you don't have a cable?

An iPhone can update itself if you have an internet connection.
Settings>General>Software Update

Settings > General > Software Update

Download & install the update. No computer required.