what do i do to trade in my mini ipad for a ipad

Apple does not have a trade-in program. You would have to check with an authorized retailer to see if they do, or sell the device yourself and use the proceeds to purchase a new device.

Depending on where you live, places like Best Buy and Amazon have or at least had programs where you can sell them your device for store credit that you can use towards the purchase of another device. How much you get depends on the condition of your device. There are also online places like Gazelle that do the same.

Something to explore (in my experience you will get less from Best Buy but you can literally walk in, trade it in and walk out in a short period of time)