My phone need software update

Assuming you have a compatible device, see: Update your iPhone, iPad, or iPod touch - Apple Support

And: If your iPhone, iPad, or iPod touch won't update - Apple Support

If the above Apple articles do not help, please post back with relevant information. 

Battery at 100% and a good strong Wi-Fi signal is needed

Tap Settings > General > Software Update

What happens?

The iPhone 7 Plus cannot update to iOS 16 or iOS 17......if that might be what you are looking for.