Select yourself, choose the band, personalize the name and click complete. If you are lucky, it won’t say that the band is out of stock and you will be back to choosing another person in the same reservation. You can use the Select button under the name of the person you want to choose the band for, even if you had already selected one for that person. Once everyone in your party has the band icon they want next to their name, you can click the purchase button. (Sorry, I do not have bands to order anymore so I cannot be sure of the words that are displayed.) If you are lucky at that point and one member in your party has an upgraded band then you will be brought to the credit card information page where you can confirm the purchase. Just make sure before you do that nobody got a grey band instead of the one they wanted.
ETA: Also, you can look for a walk-thru online with “Disney MagicBand Upgrade”.