Sprint do currently operate an Early Upgrade program.
To obtain a new phone every year through Sprint's early upgrade option, you'll be required to pay an additional $10 a month on top of your monthly service costs and phone finance payments / monthly installments.
You will need to have made payments on the phone and service for at least 12 months with the account in good standing to become eligible for early upgrade.
Any amount paid toward the principle cost of the original device would be lost and any amount still owed would be waived. Once you have upgraded to a new phone, you will be required to start over on installments for that device.
Click here to see the details of the Sprint early upgrade program for an iphone 6s