why i cant update my iphone 7 plus

Hello,

Here's two different ways to ensure the highest iOS that your device will support is on your device.

Update your iPhone, iPad, or iPod touch wirelessly. Click here --> Update your iPhone or iPad - Apple Support

Update your iPhone, iPad, or iPod touch with the Finder on a Mac computer --> Update your iPhone, iPad, or iPod touch with the Finder - Apple Support

Your iPhone 7 Plus can only update as far as iOS 15.7.3

If your iPhone, iPad, or iPod touch won't update read this support article --> If your iPhone or iPad won't update - Apple Support