it’s not letting me download the ios

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 or iPadOS.

Remember to connect to power and to turn off Low Power Mode.

Here is a link that should help:

Update your iPhone or iPad - Apple Support

If you are still unable to update, take a screenshot of the error message and post it here. Good luck!

if your iPhone is a 1st gen SE, it is not compatible with iOS 16

Identify your iPhone model - Apple Support

iPhone models compatible with iOS 16 - Apple Support

Note: if it is a first generation of SE, you won't be able to update it past iOS 15.7.2