I just bought a I phone 8 if I’m already using it how much would it cost to get a iPhone 8 Plus for a switch?

If you bought it directly from Apple, you can return it for a full refund as long as its been less than 14 days since you bought it, and pay the difference for the specific 8 Plus you want to get.

If you bought it from somewhere other than Apple directly, then their return and refund policy would apply.