How up grade iPhone Xs Max 5g

You cannot. 5G requires hardware your iPhone simply does not have.

You need special 5G antennas and cellular hardware. That’s pretty much a completely new internal logic board

..SylvieBro..

Perhaps you could see if the 'Apple Trade-in' scheme suits your need.

Though it may be OK, check for details either way; the partners used

to provide this service ~ may or not be the best way to try this idea.

[Read the fine-print of any offers; check further for more information.]

Apple Trade In - Apple (see pages for more details per this offer)

//www.apple.com/shop/trade-in

Your 'cellular service provider' may also stock new Apple iPhones;

&/or participate in similar ventures, same as authorized resellers.

https://www.youtube.com/watch?v=ncoRR2nOTLo

https://www.youtube.com/watch?v=zQcRaROi8n0