How do I update to iOS 14.0?

You don’t. You can only upgrade to the current version of ios for your device.

iPhone 8 and newer, 16.3.1

iPhone 6s through 7, 15.7.1

iphone 5s through 6, 12.5.7

When you decide to update your unidentified iPhone model, it will get the latest/available/supported iOS version.

These are the instructions:--> Update your iPhone, iPad, or iPod touch, but before proceeding, see this compatibility chart: