Iphone upgrade

Depending on where you bought it, you may be able to return it and get the iPhone X by effectively paying the difference in price. The terms of sale would dictate whether there is a restocking fee or other charges. If there is a carrier service involved that includes any promotional terms that might make the matter more complicated.

If you ordered from the Apple Store. their return policy is 14 days, however I believe they modify it during the holidays. Contact them about a return.

Contact whomever sold you the iPhone and ask what their return policy is.

You're welcome.