How can I upgrade to the 7 plus?

I recently paid off my iPhone 7 with MetroPCS however I now want the 7 plus so I called them & they said I have until the 30th of this month to get an upgrade. The employee said they wouldn't buy the phone I have though so I'm confused as to how I'm supposed to go about just paying the difference between phones

You are just buying an UPGRADE, like buying a new car without a trade in.

To you it's an upgrade, to them you are buying a new phone. That is how it works are MetroPCS, they don't do discounted phones. Or trade ins unless there's a promotion going on for trade ins.