How do I join the iPhone Upgrade Program?

Apple has provided you with the eligibility requirements to partake in their iPhone upgrade program. Apparently, per your comments, you/your phone did not qualify. To know why, you will need to contact Apple Support directly. To do so simply select the "Get Support" link in the upper right-hand side of this forum's web page.

Alternatively, you can contact Apple Sales instead: Shopping Help - Apple Store

You would have to first buy it activated under one of the program supported carriers. You can later move it to XFinity, but XFinity is not one of the upgrade program supported activation carriers. In the USA that means purchasing the new iPhone activated with AT&T, T-Mobile or Verizon. That would enroll it in the upgrade program, and later you can switch to XFinity, since the program phones are unlocked.

https://www.apple.com/shop/iphone/iphone-upgrade-program

”Works with your carrier.

You’re buying directly from Apple. Which means no more financing your iPhone through your carrier or committing to a multiyear service contract. We connect your new iPhone to your carrier for you. So you won’t have to make any changes to your rate plan. And if you ever decide to switch carriers after you activate your iPhone with AT&T, Sprint, T-Mobile, or Verizon, you can easily do that, too.“

The iPhone Upgrade Program is only on devices that have a selected carrier attached and for the monthly payment options, not the full payment option. That’s the only way the option will show for you. Simply click your current carrier, then select monthly. And the option to join iPhone Upgrade Program should appear for you. Also there will be a credit checks to determine your eligible, So be prepared to provide some information. Hope this helps!

Ok, well you will need to use the link I provided you to ask Apple to help you with getting into this program.

I used the link, and they didn’t understand either, nor did they help me sign up. They just kept asking me if my current device is eligible to upgrade (it is not because I’m not in the program yet).

Hmm that’s interesting cause I bought my iPhone through t-mobile and I can see the options but only under monthly tab.

For some time, yes. The iPhone upgrade program requires activation at time of purchase with one of the participating carriers. XFinity is not a participating carrier.