Can I change my iPhone 11 with iPhone 11 Pro Max?

If you bought your phone 14 days or less ago, you can return it for full refund, then buy the phone you want. If you've owned your phone longer than 14 days, then you cannot trade it in for a different model.

If you mean, can you use the 18W charger from the iPhone 11 Pro Max with an iPhone 11, yes.

If you’re referring to using one iPhone as a battery for another, no.

Chargers, wattage:

About Apple USB power adapters - Apple Support

Fast charge your iPhone and iPad - Apple Support

You have a 14-day return period. After that, you can always sell your iPhone 11 on your own and use the gained funds to purchase a new iPhone 11 Pro Max.