How can I update my iPhone 6 Plus to iOS 14.0?

You can't.

You cannot update an iPhone 6 Plus past iOS 12.

Apple introduced that iPhone about ten and a half years ago. It has only 1 GB of RAM. Every iPhone that can run anything higher than iOS 12 has at least 2x as much, and there are some iPhones that have 8x as much.

If you need to run a more recent version of iOS, it is past time to purchase a new iPhone. Preferably one which is compatible with iOS 18 and is likely to have years of updates ahead of it, like an iPhone 15- or 16-series phone.

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