How to change an iPhone 6 plus for an iPhone 6?

In January I got an iPhone 6 plus from T-Mobile. How can I change it for a 6 only? How can I convince my dad to change it?

Tell your dad you would like to downgrade your phone from 6 plus to a 6 to save money on paying your phone. If he asks why tell him you care for every money they spent on paying your bill… They will give in in no time…