I received an xb3 but I already have an xb6

Last week, I received an email that I needed to upgrade my gateway to take advantage of higher speeds. So, I approved the shipment. (I didn't pay attention to exactly what hardware they were sending) A few days later, I received an xb3 in the mail. But I already have an xb6 that works great! I haven't done anything with the xb3 yet. Then, yesterday I got a text saying my equipment is ready for an upgrade. If I click the link, it shows an upgrade from what looks like an xb3 to a "new modem".

I obviously don't want to keep the xb3 and send back the xb6. If I try to approve the second suggested upgrade I get an "looks like something went wrong" error.

What am I supposed to do? Why the heck did I receive an xb3? What am I supposed to ultimately have?