Hey there I want to update my iOS to 14.0

To update to the maximum of iOS 15.7.3, please 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. 

iPhone 7 can be updated to iOS 15.7.3. iOS 14 is no longer available as an update.

Make sure you have a strong Wi-Fi connection and fully charged battery.

Settings > General > Software Update