How do I change the interest split I gave for a lender?

Every lender you have answered is listed with the share of each payment that is interest. Change it by entering the interest on a recent payment from your loan statement, and DeskOps uses the new share for payments from then on. Payments that were already split keep the split they have — nothing you already booked moves without you seeing it — and the accountant's files show the interest as an expense and the rest as repaying the loan.