How do I upgrade from iPhone X to the latest model?

You buy one.

If your iPhone X is in good condition and is not carrier-locked, Apple is offering "up to $60 [USD]" in trade-in credit. I'm not sure how that might compare to trade-in deals that carriers might offer, or to selling your iPhone X once you have purchased a new phone and transferred your data to it.

Apple Trade In - Apple

Sell, give away, or trade in your iPhone - Apple Support

What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support

If you get a new phone, this article might help you transfer data from the old phone to the new one:

Use Quick Start to transfer data to a new iPhone or iPad - Apple Support

So, buy one.

Buy one. Ask the store where you make the purchase if your old phone has any trade in value.

Many carriers offer generous trade in credits.