The user agreement does not tell you how to pay the loan off early and their web site does not have an option. So I called their toll free phone and got this explaination:
1) Call their phone 888-201-6306 number.
2) Select whatever voice mail options that gets you to a Live operator - There is no explicit pay off early option.
3) Identify yourself by giving them: last 4 SS, Name and date of birth. They never asked me for Loan Number.
4) Tell them you want to pay it off NOW and to charge your credit card of record. I assume there are other methods to pay it off.
5) Wait 24 to 72 hours for transactions to clear. I suppose you can tell status of pay-off from the online account.
6) Once cleared you are free to give/sell to a 3rd party and/or buy a new iPhone on the Apple Upgrade Program.