You can upgrade your phone using your current wireless provider at any Apple store. You might want to consider the iPhone Upgrade Program, however, as that is what the article is about. Rather than pay T-Mobile monthly to lease the phone, you apply for a 0% APR CitizensBank Loan thru Apple, and pay monthly (My current payment is $41.58/mo for a 128GB iPhone 7 Plus). That payment includes the cost of the phone spread over 24 months, as well as AppleCare+ included into the monthly payments over the life of the loan. Having AppleCare+ is a huge benefit, and can save you a ton of money should you cause any damage to the phone. The program gives you the option to upgrade after just 12 months (6 months, technically, as long as you pay off 6 more months before upgrading), and comes with an unlocked device which would be beneficial should you decide to switch your carrier. Approval for this program requires decent credit and a valid U.S. Credit Card. That's right, a credit card. Not a debit card. So you'll need to weigh your options interest wise as to whether this is worth it for you. If you feel comfortable opening a 0% interest card for a promo period, do so, but beware that this will count as 2 hard inquiries on your credit score. As for availability, I didn't pre-order but waited until 12:02 A.M. to go to Apple's reservation site and luckily the iPhone 7 Plus model and carrier I wanted was available for pick-up the next day. For now, you will need to log on literally right after midnight each day to hopefully discover they have an iPhone 7 Plus available for pickup. You need to be diligent about this if you want to lock down the exact phone you want for your carrier, but I went through the process without major difficulty. These reservations, I should add, are only available to those who are interested in the Apple iPhone Upgrade Program. Using a carrier-based payment program will more than likely push your waiting period out significantly. In all, weigh the pros and cons of both and decide whether you want to cut the leasing payment cord from your wireless carrier altogether. I have attached below Apple's website to check reservation status, as well as a third-party website that is pretty accurate in determining iPhone stock availability at local Apple Stores.
https://reserve.cdn-apple.com/US/en_US/reserve/iPhone/availability?iPP=U
http://www.istocknow.com/live/