How to buy an iPhone using Apple's new iPhone Upgrade Program

Apple opened up pre-orders for the new iPhone 6S and iPhone 6S Plus on Saturday morning, but some customers are having trouble purchasing their iPhones on Apple's new "iPhone Upgrade Program."

Apple CEO Tim Cook introduced the "iPhone Upgrade Program" towards the tail end of Apple's two-hour presentation on Wednesday. It's a new system where customers can get the latest iPhone each year - plus AppleCare+ coverage - by buying an unlocked iPhone through Apple in small monthly installments, which allows customers to choose their networks at will.

If you want to buy the iPhone 6S or 6S Plus, and you want to enroll in Apple's Upgrade Program, here's what you need to do.

Most people will probably try buying their phones by going to Apple.com, visiting the iPhone section, and clicking pre-order. However, that's not how you should do it if you want Apple's iPhone Upgrade Program.

Instead, you'll want to visit the home page for Apple's iPhone Upgrade Program - just click here.

From there, you should click the option to reserve an iPhone to buy in store. You'll need to buy the phone in store on or after Friday, September 25, in order to buy the newest iPhone through this new program.

All you have to do is select your state, a nearby Apple Store, an iPhone model, and a carrier. Choosing a carrier is the tricky part, simply because some phones built for certain carriers won't work on other networks - for example, looking at the new iPhones' specs, some LTE bands on the Verizon and Sprint versions won't work on the AT&T and T-Mobile phones.

We recommend reserving your iPhone on the network you think you'll be using the most. Of course, you can discuss this with an Apple Store employee when you visit the store on September 25 or later for your reservation appointment.

Once you choose your carrier, you can choose storage and color options: The iPhone 6S and 6S Plus range from 16GB to 128GB of storage space, and you can buy it in gold, silver, space gray, and for the first time, rose gold.

Apple's website includes an important note: "Due to high demand, it's possible this product will become unavailable before you confirm your reservation." But that's okay: According to Apple, you can't change or cancel your reservation online, but if you'd like to change the model you reserved, an Apple Store employee will be able to check the availability of a different phone when you show up to the Apple Store at your scheduled time.

A few important notes about your Apple Store visit, should you choose to enroll in this new iPhone Upgrade Program:

• You'll need to bring your carrier information - account name and password - as well as any other account information.
• You'll also need two forms of identification. Apple lists the options here.
• You'll also need your current phone. Remember to back up all your data to your computer or iCloud before this appointment. If you'd like, Apple can also give you credit for your old phone - if it's an iPhone, that is - to use towards the purchase of your new phone.

Be sure to arrive at the Apple Store at your scheduled time. Apple says if you're late or miss your scheduled time altogether, it might not be able to hold the model you reserved, and you may need to make another reservation. So, best be timely! You'll be able to ask any other questions you have at your Apple Store appointment.