How to update iOS on iPhone 7 Plus?

Keep in mind, your iPhone 7 Plus no longer receives the latest iOS software updates, and cannot be updated past iOS 15.8.3. If you are trying to update to that, view the following article.

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

You can update an iPhone 7 Plus to the latest release of iOS 15 – currently iOS 15.8.3.

About iOS 15 Updates - Apple Support

Update your iPhone or iPad - Apple Support

Updating iOS: common issues and solutions - Apple Community

You cannot update an iPhone 7 Plus to iOS 16 or later. For the iPhone 7 Plus, a phone that ran iOS 10 when it came out over eight years ago, iOS 15 is the end of the line.

This may help you --> Apps that require higher iOS versions than your device can be updated to... - Apple Community