It's actually not difficult at all. Any unused training funds, even those that have been disbursed to ATP, will (and legally must be) returned to you or the lending institution.

How you finance will determine how quickly ATP refunds money. In my case, I financed through Sallie Mae, so all unused training funds were electronically transferred between Sallie Mae and ATP. The time between me informing ATP I wasn't continuing and Sallie Mae showing loan disbursements terminated was about a week. Shortly thereafter, the unused portion of the first installment was also returned to Sallie Mae.

It may be different if, for example, you take out a personal loan and are paying ATP directly from your own financial account. That said, it shouldn't be too much longer to receive your unused funds back.