if phone has trade in promo, can i pay some more money on its balance but not pay it fully yet since i dont have the funds to fully pay it in one go?

syaoran wrote:

If you pay more than what is required for the initial payment that includes any taxes and fees.  That will lower your monthly payments by whatever extra you paid spread across the 24 payments.

I am already in this deal , I was not able to add to the total amount at the time of purchase so I would like to know what I can do at this moment in time. Can I put money on it NOW after being in the deal already without being forced to pay the full balance of the phone the next month for having put more money towards the balance?