Unable to update my iPhone SE

Go to Settings

Tap General

Tap Software Update

You'll see the latest update displayed and you can follow the steps to update your phone.

Battery fully charged? Good Wi-Fi connection?

Tap Settings > General > Software Update

What happens?

If a message appears, post back with the exact text of the message.

You might also want to take a look at this troubleshooting guide:

If your iPhone or iPad won't update - Apple Support

Without knowing exactly what model iPhone you own and what iOS version is on the phone, I can only assume from your tag line that you likely own an original iPhone SE, which is now quite old and out of date. The highest iOS version a first generation SE can run is iOS 15.8.3.