How can I get iOS 15

iOS 15 is available for the iPhone 6s and later - if you have a compatible phone you can download the update in settings > general > software update.

You can also use a computer to install the update: https://support.apple.com/en-gb/HT212186

You need both a cellular connection and Wifi to update. You also need to be charged over 50% on your battery. Plug in your charger and charge your phone, better yet, leave it plugged in when you're charging.