I really want to upgrade my iPhone 7 into a 7+

If you referring to the Apple Trade-in program, based on the fact that the screen is cracked, your phone has no trade-in value. See the simulated trade-in on the following: Turn the device you have into the one you want

If you'd simply like to have the screen replaced, the cost is $149 US. iPhone Screen Repair

You can try if you get some money for your damaged 7 that you can use to get a discount on a new iPhone 7 Plus. The Apple Store is able to give you more information on how to recycle your phone.

7 Plus pricing in the US without discount:

32 GB: 569 $

128 GB: 669 $

You can't trade in a damaged phone.

Also, you cannot request a 7+ in place of your 7. If you want a different model of phone, you can sell your . phone for its parts or repair it, then sell it or trade it in.

~Lyssa