I need the iOS14.4 update for my iPhone 6 Plus

It is not possible.

Check here -> iPhone models compatible with iOS 12 -> iOS 16.

iPhone models compatible with iOS 16 – Apple Support (UK)

The iPhone 6 Plus can only go as far as iOS 12.5.6. That is the end of the line as far as updates for this model.

Frankly, you need a new phone if you want to run anything higher than iOS 12.5.6.