MPower Refinancing / Principal Repayments

Hey folks, I'm graduating this May with my MBA and have just accepted an offer.

Now that I'll have a steady income, I'm doing my finances again, because the interest rate is 15.7%, accruing daily, plus the monthly payment feels like I'm paying a second rent.

So I have to questions: 1) If I want to make a payment directly to the principal (even if its $100) does anyone know how to do it? I've scheduled my payments to go on the 15th of each month, should I do the payment on the 16th for it to be applied on the principal? Customer Service from MPower/Launch were pretty unhelpful.

2) Do you think I should go for a refinancing now, or when my salary reaches 6 figures?